Considering Hume's Copy Principle
Is all knowledge inductive?
Hume’s Copy Principle holds that all ideas we entertain are ultimately founded upon sense perception of phenomena. In Hume’s own words, “[the] creative power of the mind amounts to no more than the faculty of compounding, transposing, augmenting, or diminishing the materials afforded us by the senses and experience.” Hume’s argument is powerful; how could a blind person, without ever having experienced sight, possess an idea of seeing through pure intellection? The blind man surely cannot conceive of sight because to do so would imply not only ideation but imagination: visualizing extended substance in one’s mind. If Hume stopped here, I probably would not have chosen to respond to this prompt as I would have found doing so impossible.
Hume, however, extends his argument beyond the point of plausibility. Hume goes so far as to contend that “[a] man of mild manners can form no idea of inveterate revenge or cruelty, nor can a selfish heart easily conceive the heights of friendship and generosity.” The reductio ad absurdum of this statement would be to imply that a morally perfect person (accept the implausible conceit for the sake of the argument) is not merely incapable of conceiving how he himself could be brought to commit acts of evil but is incapable of conceiving of evil in and of itself. For example, this would be to imply that Jesus Christ—an omnibenevolent, omniscient entity in the Christian faith—is metaphysically incapable of understanding evil before witnessing it. So much for “see no evil, hear no evil, speak no evil.”
Before I present more of my own challenges to Hume’s Copy Principle, I’m going to defer to the man himself. In discussing geometry, Hume argues that the postulates upon which mathematicians deduce theories are “[themselves] owing merely to experience and all the abstract reasonings in the world could never lead us one step towards the knowledge of it.” Given my rudimentary understanding of Euclidean proofs from freshman year of high school, Hume is right thus far. For example, the fact that the corresponding angles of two intersecting lines are congruent is just that: a fact of reality, not a necessary consequence of the relationship between ideas. However, while the Copy Principle is responsible for ascertaining matters of fact, conclusions following therefrom can be drawn a priori. For example, once one has established geometric postulates, all sorts of extended conclusions can be drawn through pure intellection, i.e., deduction, about shapes, angles, etc. that the mathematician needn’t ever have perceived.
Hume argues that “knowledge of [cause and effect] is not, in any instance, attained by a priori reasoning, but arises entirely from experience when we find that any particular objects are constantly conjoined with each other.” I admit that this is true of laws of nature/matters of fact/postulates/whatever else you’d like to call those axioms which precede and predicate theoretical deduction. However, the validity and cogency of the theories deduced therefrom, so long as they do not “imply an absolute contradiction” are “not beyond the power of thought,” i.e., of pure intellection. To use another example, once a man has heard any sound whatsoever, while he cannot imagine the high-pitched tone of the dog whistle that escapes his hearing (for some physical reason I will not pretend to understand), he is more than capable of ideating such a wavelength of vibrating air particles existing, though he will and can never sense-perceive it, nor can he compose it from sounds he has observed.
Hume’s Copy Principle accurately describes how postulates are formed but it fails to prove that no concept can be deduced a priori.