David Hume, a Scottish philosopher of the 1700s, wrote about the logical relationship between ideas, the nature of the senses, causality, and skepticism in his 1748 book An Enquiry Concerning Human Understanding. As far as what he puts forth that is rational, all of this I knew (and others can know) due to reason before or aside from having read or heard of Hume. There are many everyday sensory experiences that could prompt someone to realize that it does not logically follow from one event correlating with another that there is a causal relationship, and it is apparent from conversation that many people seem to confuse a priori knowledge, or knowledge of logical necessities, with sensory experiences and fallible evidences, which are absolutely distinct. These things are, after all, true by the logical necessity that Hume selectively relies on and dismisses as assumed or trivial. His skepticism is nevertheless sometimes entirely correct, even if he couches it in a blatantly irrationalistic ideological context.
Early in this writing, he distinguishes between the relations between certain ideas, such as how five times three is 15, or half of thirty, and what he calls "matters of fact," which are really about the nature of the sensory world and the exact relationships between objects and events. There is a great deal he leaves unmentioned, if he had ever discovered or accepted it to begin with, about logical axioms and other necessary truths, which are what makes mathematical and geometric truths fixed and absolutely certain. The very poor phrasing on Hume's part aside, for logical truths are matters of fact that any empirical fact would hinge on, he quickly and directly admits that there is no absolute certainty about empirical matters many utterly take for granted:
"That the sun will not rise to-morrow is no less intelligible a proposition, and implies no more contradiction, than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply a contradiction, and could never be distinctly conceived by the mind." (15-16)
Ask many people, and they would say that of course the sun will rise tomorrow, and this is absolutely certain. Why would anyone think this? Only on the basis of being told by others or of remembering previous times the sun ascended, and then extrapolating via the inductive fallacy to the day to come, would anyone be hold this belief. It is a total assumption. Yes, the sun either will or will not rise tomorrow, but not both at once, and each option is logically possible because it does not contradict logical axioms (not that Hume goes so far as to mention the intrinsic truth of axioms). As probable as it seems from the sensory and memory evidence, the belief that the sun will rise tomorrow is irrational because there is no logical necessity in this, as not even a past empirical pattern requires a continuation of that pattern into the future. Hume then moves on to other issues of sensory epistemology and metaphysics, focusing on the indirect nature of things like inferences and hearsay:
"If you were to ask a man, why he believes in any matter of fact, which is absent; for instance, that his friend is in the country, or in FRANCE; he would give you a reason; and his reason would be some other fact; as a letter received from him, or the knowledge of his former resolutions and promises." (16)
Though Hume does not tackle this on particular, you already cannot know if you are in this country or that country due to the logical possibility of sensory illusion, only that it seems as if you are, so geographical location would certainly not be knowable about other people who are out of sight, whose consciousness one does not directly experience and who could be lying or lied about as far as hearsay goes. Someone could still believe their friend is in another country on the basis of his or her letters, but they would not rationally believe this. The indirectness of such hearsay is emphasized by Hume as being inferred and neither known by logical necessity nor perceived even by direct sensory experience. He soon writes on how cause and effect permeates perceptions of external events, like how hearing a voice in the dark might lead someone to think the sound is caused by another human. No such thing is believed on a priori grounds, Hume acknowledges, and only because of experiential familiarity and custom would anyone believe otherwise. His example of an unfamiliar item being placed before someone's eyes rightly presents how no one with human limitations could truly know the functions and correlations of a material object just by thinking about what they see:
"Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects." (17)
Now, even after thorough experience, one could not know the item's true nature beyond that it is by necessity consistent with logical axioms (being inherently true, axioms govern all things and thus nothing could be outside of or contrary to them), only that the senses have made it appear that a given seeming cause is correlated with an effect, or that it seems to have a certain shape, color, and texture. However, no one could just look at a truly unfamiliar sensory object and know by self-evidence or by logical necessity what its functions are. This knowledge, restricted to the level of mere perception, is prompted by experience rather than strictly looking to reason, but Hume does not address how logical necessity and possibility metaphysically dictate all things and can never be epistemologically escaped.
In reality, the idea that one specific, observed material event has certainly caused another is a delusion that only non-rationalists or partial rationalists--irrational people to different extents--would ever believe. Hume uses the examples of the explosion of gunpowder and the magnetic attraction of a loadstone (spelled lodestone in contemporary American English) to discuss how these and similar things are not discoverable "a priori." Moreover, Hume asks, "Who will assert, that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or tiger" (17)? No one could know even if they had full awareness on the level of mere perceptions as to what outcomes result from specific creatures consuming exact amounts of particular foods and their nutrients! Even so, it is far from obvious ahead of time which foods would likely be eaten by which animals, and yet even direct, repeated observation does not establish if scientific laws or sensory norms will persist.
An Enquiry Concerning Human Understanding. Hume, David. Indiana: Hackett Publishing Company, 1993. Print.
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