Philosophical Bridges
A bridge is a structure which you use to cross a chasm in order to get the other side. Two things make a bridge good, its prudence and its usefulness:
1) prudence (the bridge goes somewhere it should)
2) usefulness (its durability and capacity)
In philosophy, Hume argued that there is no way to bridge the connection between how things appear to us and how they really are. Here is his entire argument, distilled down to its essentials, first from A Treatise of Human Nature [1]:
All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call impressions and ideas. ... perceptions, which with the most force and violence, we may name impressions; and under this name I comprehend all our sensations, passions and emotions, as they make their first appearance in the soul. By ideas I mean the faint images of these in thinking and reasoning. ...
... nothing is ever present to the mind but perceptions ...
We have no other notion of cause and effect, but that of certain objects, which have been always conjoin'd together, and which in all past instances have been found inseparable. ...
Thus all probable reasoning is nothing but a species of sensation. 'Tis not solely in poetry and music, we must follow our taste and sentiment, but likewise in philosophy. ...
... necessity is something, that exists in the mind, not in objects; ...
... all knowledge degenerates into probability ...
If we believe, that fire warms, or water refreshes, 'tis only because it costs us too much pains to think otherwise. ...
... errors in religion are dangerous; those in philosophy only ridiculous. ...
Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them. ...
And then from An Enquiry concerning Human Understanding [2]:
One event follows another; but we never can observe any ties between them. They seem conjoined, but never connected. And as we can have no idea of any thing which never appeared to our outward sense or inward sentiment, the necessary conclusion seems to be that we have no idea of connexion or power at all, and that these words are absolutely without any meaning, when employed either in philosophical reasonings or common life.
As you can see from above, Hume thought of the mind as a sense organ--a passive detector of fluctuation coming in from our ambient array of sensory stimuli. On this view, you can never determine cause and effect, and all you can do is to go on recording the fluctuations coming in (making an ultimately inconsequential mental note of the regularities you experience).
These ramblings of Hume awakened Kant from his "dogmatic slumber" because Kant could see how some extremely useful things (Newtonian mechanics, Euclidean geometry, etc.) would then get de-legitimized if Hume's ramblings were to be adopted by humanity. This would leave humanity worse off.
In a vain effort to save these extremely useful things from Humean destruction, Kant erroneously accepted Hume's thinking errors as representing the truth, and then went on to create a "backstop" which would allow for mechanics and geometry to continue to be useful to humankind--even if the mind is a sense organ, only (with no conceptual powers of awareness).
Empiricists had been stating that knowledge must conform to experience, but Kant flipped this around and said that our experiences conform to knowledge [3]:
Hitherto it has been assumed that all our knowledge must conform to objects. ... We must therefore make trial whether we may not have more success in the task of metaphysics, if we suppose that objects must conform to our knowledge. ...
What objects may be in themselves, and apart from all this receptivity of our sensibility, remains completely unknown to us. ...
It is ... solely from the human standpoint that we can speak of space, of extended things, etc. ...
Thus the order and regularity in the appearances, which we entitle nature, we ourselves introduce. ...
It is therefore correct to say that the senses do not err--not because they always judge rightly, but because they do not judge at all.
To accomplish such a wide-scale task (of rescuing humanity from Humean destruction), Kant created ontological idealism--a brand new way of thinking. Subsequent philosophers were then mesmerized by Kant's elaborate schematism, which separated reality into two realms: phenomenal and noumenal.
According to Kant's elaborate schematism, humans are stuck in the phenomenal realm of experience, like the chained man in Plato's Allegory of the Cave--only ever able to see shadows on the wall, without ever actually experiencing the objects which cause those shadows.
Kant's "improvement" with regard to Plato's cave was merely to claim that certain categories, things such as space and time, are "hard-wired" into human thinking. In other words, merely by thinking, we are helping to create the shapes of those shadows on the wall!
But this leaves the noumenal realm--the realm of real existence (of "things-in-themselves")--outside of all human understanding. Kant didn't unchain the man in the cave, and then take him out of the cave so as to show him which actual objects had been creating the shadows.
Instead, he put extra chains on the man, and consoled him by convincing him that, though watching shadows is all he will ever be capable of, he can take a least some comfort in the fact that the shadow shapes are coming from innate ideas already contained within his own mind.
The shadows are still just shadows, but they are invariant shadows that all humans "create"--so that we can usefully form and use theories from these shadows (theories such as Newtonian mechanics), even though the shadows are not actually "reality."
In 1974, Ayn Rand gave an address to the US Military Academy at West Point [4] which got published a decade later as the first essay in a book of the same name (Philosophy: Who Needs It?). In the address, she started with a thought experiment of an astronaut stranded on an unknown planet. She said such an astronaut would have 3 first questions:
1) Where am I?
2) How can I discover it?
3) What should I do?
She went on to say how we are all faced with the same 3 questions, and how the various philosophies answer these crucial questions (errors to which Hume would call ridiculous, but not "dangerous"!), and how those answers create consequences in our lives.
She knew of the danger of wrong answers to the fundamental questions which all humans must face up to in their personal lives. Her emphatic critique of Kant--though often filled with so much animosity it makes people feel it might not be a fully objective account of Kant's "contributions" to human philosophy--is one of her major contributions.
According to Rand, Kant created a philosophical bridge, but it actually was an unneeded "bridge-to-nowhere" [5]:
Even apart from the fact that Kant’s theory of the “categories” as the source of man’s concepts was a preposterous invention, his argument amounted to a negation, not only of man’s consciousness, but of any consciousness, of consciousness as such. His argument, in essence, ran as follows: man is limited to a consciousness of a specific nature, which perceives by specific means and no others, therefore, his consciousness is not valid; man is blind, because he has eyes—deaf, because he has ears—deluded, because he has a mind—and the things he perceives do not exist, because he perceives them.
Reference
[1] Hume, David. (1739-40). A Treatise of Human Nature. [ed. L. Selby-Bigge]. p 1-415.
[2] Hume, David. (1748). An Enquiry concerning Human Understanding. [ed. L. Selby-Bigge]. p 74.
[3] Kant, Immanuel. (1781). Critique of Pure Reason. [trans. N. Kemp Smith]. p B xvi and also A 42/B 59 - A 293/B 350
[4] Rand, Ayn. (1984). Philosophy: Who Needs It? New York, NY: Signet. p 1.
[5] Rand, Ayn. (1961). For the New Intellectual. New York, NY: Signet. p 32 (paperback edition).
*Image at top taken from: loc.gov/free-to-use/ [found at: cdn.loc.gov/service/pnp/ggbain/16800/16889v.jpg]