The invisibility is commonly attributed to abstract notions. The invisibility seems the simplest thing to be said about them and, accordingly, these notions shows themselves as the simplest objects of knowledge when we point out to their invisibility (for instance, the simplicity of numbers, the moral and esthetical concepts, etc.). However, as long as they are not put aside by claiming their invisibility, such property proves to be, on the contrary, one means for enhancing the complexity of the abstract notions.
First of all, for the most visible objects possess also the property of being simple, simpler than abstract and invisible ones. Though we always see the objects in a landscape or among other things, they can easily be detached from the rest. Moreover, the multiplicity of visible objects stops any profound inquiry in their nature: we just see that tree, and not the other. The emphasis lays on the difference of a visible thing from other visible things, not on the thing itself, in order to reveal its complex set of peculiarities.
An unseen object has not the same possibility of being accepted with the easiness met when we observe visible objects. This fact may be elucidated as long we regard to the unseen aspects of the concrete objects. A thing that conceals some of its sides is recomposed in its entirety by an appeal to our memories about the similar things viewed before (such explanation was offered in ancient medical writings). Of course, the act of recomposing means an implicit admission of its complex nature.
The case of an unseen person brings on more details about the complexity of invisible objects. When we do not see a person, we do not consider her as totally inexistent or as possessing a simple existence as any other object put aside from our sight. An unseen person is thought of as existing in a complex manner, as being caught in her activities, places, or things possessed. When we hardly can imagine such complexity, we tend to fill the gaps with general considerations supposed to be adequate to that person, too. Thus, we may suppose that the unseen person does what other persons do in the same conditions, for instance, if someone is a fisherman, he is pictured as fishing, and if someone caused an injury to other people, he is thought of as doing all the wrong deeds we generally attribute to a so-called evil person.
In short, the unseen persons involve for our thoughts about them the two tendencies of placing them in complex states of facts and also to include them in general classes.
The complex discourses and relations generally associated to abstract notions and the operations of classifying them prove that the above mentioned tendencies are followed in their case, too. Nonetheless, the appeal to memory for building abstract notions is a common manner of dealing with them, as much as we do this regarding partially unseen objects.
When we appeal to memory for knowing the unseen sides of a thing, we are far from it or we won’t to make the effort to see it completely. An unseen person imagined in the midst of a lot of things is usually one we want dispersed and lost in those things in order to not bother ourselves with the sober matters of thinking about what means the absence of others. Moreover, her inclusion in categories provides us the means to forget her as an individuality.
Surely, all the above considerations about the unseen objects and persons may be also identified in the mental processes involving abstract notions.