I have pointed out a number of times that although sensation as such belongs in Wolff's psychology to the less clear side of faculties, even sensations can be more or less clear. Just consider a common enough experience, such as perceiving a bicycle. When we notice a lone bicycle, the actual sensory data received by human mind contains a lot more than just the bicycle, for instance, balcony above the bicycle and bricks on the wall that the bicycle is leaning against. Then again, what we are clearly aware of includes only a fraction of this data, namely, just the bicycle, while the wall, the balcony and others recede into a murky background. This effect can be very pointed, as shown by the famous example of people counting how many scores a player makes in a basketball match not noticing a guy in a monkey suit dancing on the field. This difference in the levels of clarity Wolff considers under the concept of attention. We might say that if human consciousness is like a light, only some objects can be in its focus.
The effects of the faculty of attention seem not completely positive, because the necessity of focusing one's attention on a part of sensory data prevents the possibility of considering more than one thing at a time. This is especially lamentable in case of sensations, which are easily disturbed by other sensations. Thus, it is not easy to concentrate one's attention on some particular sensation for a long period of time, because other sensations constantly demand our attention too.
Yet, although attention itself has a limited range and might thus not show all facets of a thing, we can also systematically move our attention from one facet of a thing to another and thus ultimately go through it all – this method Wolff calls reflection. We can also imagine the various parts of the thing as separate from the whole – this is what is called abstraction. When we then reflect how the various abstracted parts combine into a totality, the result is a more detailed view of the structure of a thing, which is not just clearer, but also more distinct, due to us having discerned the various parts of the thing and their interrelations and retaining all of this in memory.
Now, this stage of representing things distinctly is already intellect, Wolff defines, thus further confusing the lines between sensational and intellectual faculties – looking at a particular bicycle and seeing how all its parts combine to form a complex machinery that will move the one riding forward is already work of intellect. In fact, this is also an instance of intuitive cognition, by which Wolff means cognition generated immediately by examining our ideas – such cognition can be confused, if we don't know anything about the structure of what we examine, but through reflection it becomes more distinct. Thus, Wolffian psychology allows for the possibility of intuitive intellect, although this notion has a completely different meaning than with Kant (I assume the awareness of the mechanics of a bicycle wouldn't be something Kant would call intuitive intellect).
Note that the use of intellect is not restricted to mere universalities, but intellect could be applied individual things, like bicycles. Still universals are a topic studied by intellect – we can not just reflect on parts of a thing, but also on similarities and differences between different things, thus becoming aware of universals or features shared by many things. Moving to universalities usually requires the use of words that can be used to symbolise individual things and especially universalities – the use of words is then properly called symbolic cognition. Although symbolic cognition thus makes it easier to reflect on universal features of things and might help us to invent new things (especially through some Leibnizian ars characteristica) and is especially useful in communication of ideas, we can always refrain from using it and remain on the level intuitive cognition, Wolff assures us.
While the level of intellect as such is already achieved, once we have distinct ideas, it is of course possible to have more distinct ideas (e.g. to know what the parts of the bicycle are made of). This possibility implies a final level of highest intelligence that has nothing but distinct ideas of everything (note that this would essentially require an infinity of ideas, because the things that we perceive are infinitely divisible). A notion distinct from the idea of a highest intelligence is that of a pure intelligence. Pure intelligence does not so much know everything perfectly, but is undisturbed by various sensations and uncontrolled imaginations that distract human attention so easily.
So much for a general look on reflection and intellect, next time I shall look more closely at the use of intellect.