in these matters except through analogical inference. Knowledge of previously unknown particulars existing in external reality is thus attained by analogy with those already known. This is a truth that no rational person disputes. Indeed, it is among the most distinctive features of the intellect that set it apart from sense perception: sense perception apprehends only particulars, whereas the intellect apprehends things in a universal, unrestricted form, but through analogy. The intellect then apprehends all these universals even when the particular examples are absent from its awareness. Originally, however, they became general universals in the mind only after it had conceived particular examples of their individual instances.

When a long time has passed since the mind considered those particular instances, it may frequently err by making its judgment either too broad or too narrow. This often happens to people: a person supposes that the universal propositions he holds are sound, whereas scrutiny reveals otherwise. They conceive something intellectually, and what they conceive is an object of intellectual apprehension. They then discuss it, imagining that they have discussed an essence abstracted in itself, considered simply as such, without its existing either in external reality or in the mind. Thus they speak of:

“The human being as such,” “existence as such,” “blackness as such,” and the like.

They imagine that this essence, which they have stripped of all negative and affirmative qualifications, is realized in external reality in this abstracted form. This is an error like that of their predecessors concerning the numbers, Platonic Forms, and other things they abstracted. Rather, these abstractions are only posited in the mind. Not everything the mind supposes can exist in external reality. This is what is called mental possibility. For possibility is of two kinds.