Crusius seems to be going through different philosophical disciplines somewhat systematically. He started from practical philosophy, moved on to metaphysics and Weg zur Gewißheit und Zuverlässigkeit is a book on logic.
Just like Wolff, Crusius begins his treatise on logic with a preliminary study of philosophical disciplines. First, he defines philosophy as the sum of such cognitive states that have to do with truths of reason that have a constantly enduring object. Here, the truths of reason, he adds, are such that can be known from consideration of natural things, opposed to truths of revelation. Philosophical truth, as Crusius understands it, is such that can be known through mere reason and that must have an object that is either simply necessary and unchangeable or endures constantly in the current world so that it cannot naturally stop existing. Philosophy should then deal with the essence of things and the causes of what we perceive, Crusius thinks, because these are constant. Individuals, on the other hand, might not be, but if they are, they also are among objects of philosophy. Then again, variable individuals are dealt in philosophy only, if it sheds light on some constant objects.
Crusius combines this concept of philosophy with the goal of human nature in order to determine what good philosophy is like. Firstly, he says, good philosophy must be true and grounded in distinct concepts, correct proofs and insight about the true relations of things. Distinctness, Crusius adds, does not mean understanding something, but only an ability to distinguish the topic of the concept from other topics. Similarly, he adds, correct proofs do not always require geometric certainty, but probability suffices. Insight, finally, is not necessarily linked to any single method, according to Crusius. In addition, he states, good philosophy should not concern mere common and familiar things and also not mere perceptions of existence, but try to find their real grounds. The final requirements for good philosophy are that it is useful and sharp in the sense that it comes with a distinct representation of the manner in which it has been abstracted.
Just like many philosophers of the time, Crusius differentiates between philosophical and historical knowledge. Philosophical knowledge, he says, explains the grounds of things, while historical knowledge concerns only events in their existence. Philosophy does not exclude all historical knowledge, Crusius explains, because it should concern the existence of all unchanging things, but it cannot show the real ground for non-sensuously existing things, being able only to prove their existence. Then again, he adds, while philosophy is not opposed to historical knowledge, it is opposed to history of changing things, although this can be a means for knowing philosophical truths. Furthermore, even philosophical truths can be known historically, if one does not know how they are proven, while one can have philosophical knowledge of changeable things, if one knows their ground. Thus, Crucius concludes, philosophy is not identical with philosophical knowledge.
Crusius divides objects of philosophy into magnitudes of extension, which are dealt by mathematics, and anything else, which is dealt with philosophy in the strict sense of the word. Mathematics he particularly divides into pure mathematics studying magnitudes in abstract and applied mathematics studying magnitudes concretely in certain natural things. He then goes on to divide pure mathematics into arithmetics or study of discrete magnitudes, geometry or study of continuous magnitudes and algebra or study of magnitudes in general. Applied mathematics, Crusius states, could have an infinitely many kinds of objects, but as a part of philosophy it should study only unchangeable natural objects (thus, architecture, for instance, is not a part of philosophical applied mathematics, although its method would be philosophical). The proper parts of applied mathematic, he insists, fall into two broad categories: astronomical sciences dealing with magnitudes abstracted from celestial bodies (including Earth) and sciences of movements of natural bodies, such as mechanics and optics.
Crusius notes that mathematics has often been distinguished from philosophy proper, although the latter does not have any peculiar object of its own, but deals with everything that is not mathematical. He suggests that this habit implies that there must be more to this distinction, in other words, philosophy in a strict sense should be studied in a different manner from mathematics. Crusius begins to search for this distinction by thinking about the object of mathematics or quantities as quantities. In order to enable studying quantities, mathematics has to assume that nothing else is variable but the magnitude of the objects it studies, while everything else we perceive in these objects must be essential. In other words, he explains, the objects studied in mathematics must be so simple that change of any non-quantitative feature or quality would make it a different entity (for instance, a triangle that can have only a certain number of sides). This is something not true of objects of philosophy, which might change e.g. their figure without changing their essence.
Because the objects of mathematics are so simple, Crusius argues, a mathematician can take just a single thing and abstract from this one instance a definition that applies only to this kind of object. Thus, he adds, a mathematician could also define quantities by telling how they are generated, which is not true of philosophy. Then again, Crusius thinks that mathematics does not study goals or even causes, while in philosophy the essences of things might depend on goals and causes.
When it comes to mathematical methods, Crusius suggests, mathematics rarely divides things in species, since its objects are usually too simple for divisions. Because of this, he continues, mathematics rarely uses disjunctive proofs. Especially pure mathematics, on the other hand, has to use the strongest form of deduction or demonstration. This is connected to the fact that pure mathematics is heavily dependent on the proposition of contradiction, and indeed, Crusius thinks, it should use no other principles, because it investigates such abstract topics that could be understood from the definitions with the help of the principle of contradiction. Philosophy proper cannot do this, he adds, when studying e.g. concrete causes. Crusius discovers also a more logical feature characterising mathematics. That is, he says, mathematical propositions, where one quantity universally determines another, can be converted, so that the second quantity determines also universally the first quantity, while in philosophy proper converting a universal proposition leads in general to a mere particular proposition.
From mathematics, Crusius turns to the division of philosophy proper. He begins by pointing out that some objects of philosophy proper are simply necessary or at least must exist in every possible world, while others are contingent in the sense that they could fail to exist, even if some world is assumed to exist. Crusius still does not want to divide philosophy into necessary and contingent parts, because in case of practical truths this division would cause difficulties. Instead, he advocates dividing philosophy into metaphysics, studying necessary theoretical truths, and disciplinal philosophy, studying both contingent and practical truths. Proceeding to metaphysics, Crusius divides it into ontology, studying the general essence of things in general and their a priori distinctions, theoretical natural theology, studying God, together with divine properties and actions, and cosmology, studying the necessary essence or world in general and everything derived from it a priori. He also mentions as an important part of cosmology the metaphysical pneumatology, studying the necessary essence of spirits.
Crusius divides the objects of disciplinal philosophy into three kinds: bodies, nature and use of understanding and truths pertaining to human will. The first type or the bodies, he says, is studied by physics, which reduces experiential truths about bodies to their causes and uses both to deduce more of their properties and effects. Crusius notes that physics contains innumerably many sub-disciplines, but that the physics proper deals with the most general truths concerning bodies. He also mentions as more particular parts of physics the medicinal sciences and teleology as the study of purposes of natural bodies.
Crusius is very brief on logic: he has plenty of time to delve on that topic later on. For now, he merely defines logic as the study of capacities and effects of understanding and its use in knowing truth and mentions that sometimes noology is separated from logic as an independent discipline studying the essence of understanding.
The final object of the disciplinal philosophy or human will, Crusius states, is studied by two sciences, thelematology and practical philosophy. Of these two, he explains, thelematology studies the essence, natural capacities and properties of will, while practical philosophy studies rules for directing its actions and passions. Crusius insists that unlike e.g. Wolffians had done, thelematology and the previously mentioned noology should not be classified within metaphysics, since neither of these two disciplines deals with necessary things.
Moving on to practical philosophy, Crusius divides it to natural right in the widest sense, studying human will in relation to divine natural laws, and prudence, studying human will in relation to its own purposes. He insists that the two disciplines should not be confused, although it has been done often, starting from ancient philosophy, so that the drive for happiness has been seen as natural law. Crusius further divides natural right into general practical philosophy, studying grounds of obligation and their consequences, kinds of duties and grounds of their application and collisions, and more determined disciplines, explaining main classes of duties, including ethics, studying virtuous direction of one’s own mind and other conditions, natural moral theology, studying immediate duties toward God, and natural right in the strict sense, studying duties of humans toward one another. The natural right in the strict sense, he notes, divides into universal public right, studying duties of rulers and subjects toward one another, and universal right of nations. Because natural laws are divine laws, he adds, all the parts of natural right in the wider sense form the practical part of natural theology. Finally, Crusius divides prudence into general prudence, studying general readiness of reasoning spirit to choose and apply means appropriate for goals, private prudence, studying goals of individuals, and state prudence, studying goals of community.
Crusius notes that he has used different words for the various divisions of philosophy, such as theory (Lehre) and science (Wissenschaft). He explains that science, when it is understood objectively and not as a capacity of understanding, means such a sum of scholarly truths that is of remarkable extension and that is collected together because of some reason, because it wouldn’t make sense to deal it as one whole without any reason to do so. Crusius adds that it is indifferent to science, what kind of proofs are used in it, as long as they are correct in their kind. By theory, on the other hand, Crusius means any piece of knowledge that is remarkably raised above common knowledge, whether according to matter, that is, by concerning things that could not be known without purposefully learning about them, or according to kind of knowledge, that is, as being logically better arranged than common knowledge.
Every science, then, has a reason or ground why this collection of truths is taken as a whole. Crusius divides these reasons into four kinds. First reason is that the truths of a science come under a common concept, which has species or individuals that we want to study: examples of sciences based on such a reason are physics and geometry. Second reason is that the truths of a science are parts, determinations or consequences of a real whole. Crusius suggests as examples in this case physiology and thelematology. He includes under this case also sciences that are further determinations of a concept that earlier was thought concretely and indeterminately, like with logic. The third reason is that the truths are derived from a general and determinate fundamental proposition, like with natural right, and finally, the fourth reason is that the parts of a science relate like means to an end, like in ethics and algebra.
According to Crucius, reasons or grounds of science should be adequate. He suggests several rules for deciding this. Firstly, Crusius says, a science should be extensive enough. This means, he explains, that it must have enough truths going beyond common knowledge, and if it is a philosophical science, it should study things that endure or that have principles that endure. Crusius notes that this criterion depends on the intended purpose of the science: according to some purpose, a science should be divided, while with another purpose it should be combined with other sciences. Thus, he confirms, the division of truths into different sciences should produce real usefulness. Then again, Crusius adds, if there is no reason important to do otherwise, one should not depart from common divisions of sciences. A possible important reason might be, Crusius suggests, if an essential class of truths would not be sufficiently perceived or if otherwise such truths would be confused. Finally, he notes, application of these rules is based on postulates that can be perceived, but not clearly proven, thus, there is much arbitrary in determining number and limits of sciences.
Crusius notes several consequences of his account of the rules for dividing sciences. Firstly, he says, even if these rules are followed perfectly, there still might be special needs for individual treatises having their own themes. Even further, Crusius says, for a special purpose, an extensive theory could be ordered differently than would be naturally correct. Finally, he notes, since sciences often depend on other sciences, there isa need for encyclopaedias that order sciences hierarchically.
Crusius envisions two reasons for studying philosophy. Firstly, he says, philosophy is useful in its matter, that is, the determined truths of philosophy, because truths about constant things can be used in all sciences as axioms for correct proofs. Indeed, Crusius adds, even truths about variable things are grounded in constant truths. The second reason to study philosophy, he continues, is that a thorough examination of philosophy leads to a cultivation of understanding philosophy, at least if philosophy is not studied just historically. Thus, Crusius thinks, although metaphysics is the first a priori science and physics the first a posteriori science, the study of philosophy should still start from logic.
Crusius notes that many oppose the notion that philosophy is useful and might even despise it. Sometimes, he explains, this attitude might be caused by its disordered state. A further reason, Crusius suggests, is that it is thought to be dispensable, because there are practically learned people, who know no philosophy. Furthermore, he adds, some might think philosophy is unuseful, because they suppose it cannot be applied in practical life, because its methods cannot be applied anywhere beyond philosophy, because philosophers disagree so much with one another and because there are philosophers who have had no success in practical life. Crusius notes that some think studying philosophy is even shameful, because it is a waste of time and a source of errors even in practical life.
Crusius admits that philosophy as precise knowledge of unchangeable truths of reason is not necessary for all professionals or even for all scholars, because human understanding does have a natural feeling of true and false. Still, he insists, philosophy is useful for the two earlier mentioned reasons that it teaches truths and improves our understanding. This indicates, according to Crusius, a moral necessity to learn philosophy, if one just is capable of this. Furthermore, he continues, one demands too much of philosophy, if one thinks that theoretical knowledge of philosophy alone should be enough for practical professions, when they still require more particular disciplines and even historical knowledge. Indeed, Crusius says, all useful scholarship requires well-based theory and many-sided exercise. He also points out that faults found in theories of a specific philosopher do not touch philosophy in general: it is not the fault of philosophy, if someone uses it incorrectly.
Still, Crusius thinks, philosophers themselves have a lot to blame for their negative reputation, since its history is a story of errors. Philosophy has to be dealt with carefully, he emphasises, but adds at once that this is taught by philosophy itself, especially in logic. Crusius admits that one still needs other means to avoid faulty philosophical concepts, in order to avoid partial bias, or one has to train one’s mind to be able to appropriate truth. This implies, he explains, that one should get used to paying attention to things, one should learn to be patient and diligent in order to not stop working too early, one should learn to be impartial, modest and prepared, one should not be too sceptical nor too eager to believe, one should less contradict others than to search for the truth, since disputation does not lead to anything positive, and one should be virtuous, in order to be worthy of divine providence.
Crusius suggests investigating the relation of philosophy and Christian theology, because they are often depicted as and sometimes are enemies, but need not be. He begins by noting that it is false to think that philosophical knowledge could generate a belief in the revealed truths, Furthermore, Crusius adds, philosophy cannot be used to gain the approval of the opponents of Christianity through the refutation of their errors. Similarly, he notes, based on revelation, philosophy cannot make people at current stage of humankind truly virtuous. Crusius also thinks that philosophy is not indispensable for theology, because a theologian can also use natural abilities of understanding. Indeed, he suggests, the danger of abusing philosophy in relation to theology is greater than in relation to other truths.
What reason is left for the theologian to learn philosophy? Crusius states that revelation does not free us from the duty to cultivate our understanding and to know God from divine works. Furthermore, he adds, philosophy helps to make theology more ordered and well founded and to show why objections against it are unfounded. Crusius also thinks that it shames atheists to see that believers can be sharp thinkers. Finally, he insists, study of theology becomes systematic through the habit of sharp thinking. Crusius emphasises that all heresies are not born of philosophy, since they can be also generated by lack or misuse of philosophy and not by true philosophy. Still, he thinks, one has to be careful in applying philosophy to theology, since the former often uses without restrictions propositions that need restrictions in the field of theology.
Crusius also thinks that philosophers can gain advantages from Christianity. Firstly, he says, Bible contains truths of theoretical natural theology and practical philosophy and of true historical conditions of the origin of the world and of its current state, thus, a philosopher could learn from it many truths they otherwise wouldn’t know. Furthermore, Crusius adds, a philosopher can find errors in their conclusions, if they find them contradicting with the Bible. He answers the objection that theological truths could not be philosophical by insisting that no matter what the source of true proposition (e.g. testimony of others or our own ingenuity), it can still be philosophical, as long as a constant ground for it could be found from the observation of natural things.
Crusius concludes the chapter by listing the most important and most general means for learning philosophy. First and foremost, he begins, one should search for a capacity to increase one’s capacity to learn, which means studying logic. Then, he continues, one should get as complete a concept of the whole area of philosophy as possible, because sciences hang so closely together that not knowing one can lead to errors in understanding others. Furthermore, Crusius says, one should get used to attending to everything one observes, since good attention can always reveal opportunities to learn new things or to make old truths more distinct. He warns the reader not to learn everything by oneself, because that leads usually to uniform understanding and one-sided knowledge of things, and in order not to rely just on one’s own meditation, one should read texts of others, especially when one is just starting to study. He also emphasises that these guidelines should be followed constantly, in order to make them a habit. Crusius especially underlines the need to study history of philosophy, because it is important to learn about possible theories and to show the importance of being careful, and to study mathematics, because it trains understanding and it helps to show the difference between mathematics and philosophy proper.
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