Now I assert, that the faculty manifested in the acts here narrated does not differ _in kind_ from Understanding, and that it _does_ so differ from Reason. What I conceive the former to be, physiologically considered, will be shown hereafter. In this place I take the understanding as it exists in _men_, and in exclusive reference to its _intelligential_ functions; and it is in this sense of the word that I am to prove the necessity of contra-distinguishing it from reason.
Premising then, that two or more subjects having the same essential characters are said to fall under the same general definition, I lay it down, as a self-evident truth,--(it is, in fact, an identical proposition) that whatever subjects fall under one and the same general definition are of one and the same kind: consequently, that which does _not_ fall under this definition, must differ in kind from each and all of those that _do_. Difference in degree does indeed suppose sameness in kind; and difference in kind precludes distinction from difference of degree. _Heterogenea non comparari, ergo nec distingui, possunt._ The inattention to this rule gives rise to the numerous sophisms comprised by Aristotle under the head of μεταβασισ εις αλλο γενος, that is, transition into a new kind, or the falsely applying to X what had been truly asserted of A, and might have been true of X, had it differed from A in its degree only. The sophistry consists in the omission to notice what not being noticed will be supposed not to exist; and where the silence respecting the difference in kind is tantamount to an assertion that the difference is merely in degree. But the fraud is especially gross, where the heterogeneous subject, thus clandestinely _slipt in_, is in its own nature insusceptible of degree: such as, for instance, Certainty, or Circularity, contrasted with Strength, or Magnitude.
To apply these remarks for our present purpose, we have only to describe Understanding and Reason, each by its characteristic qualities. The comparison will show the difference.
1. Understanding is discursive. 1. Reason is fixed.
2. The Understanding in 2. The Reason in all its decisions all its judgments refers to appeals to itself, as the ground some other Faculty as its and _substance_ of their truth. ultimate Authority. (Hebrews vi. 13.)