our manner of knowing is so weak that no philosopher could perfectly investigate the nature of even one little fly.

Any fool can see the limits of seeing, but not even the wisest know the limits of knowing. Thus is ignorance rendered invisible, and are all Men made fools.

We are not at all to wonder [...] that we having but some few superficial ideas of things, discovered to us only by the senses from without, or by the mind, reflecting on what it experiments in itself within, have no knowledge beyond that, much less of the internal constitution, and true nature of things, being destitute of faculties to attain it.

Man's knowledge, save before his fellow man,
Is ignorance—his widest wisdom folly.

It always seems to me extreme rashness on the part of some when they want to make human abilities the measure of what nature can do. On the contrary, there is not a single effect in nature, even the least that exists, such that the most ingenious theorists can arrive at a complete understanding of it. This vain presumption of understanding everything can have no other basis than never understanding anything. For anyone who had experienced just once the perfect understanding of one single thing, and had truly tasted how knowledge is accomplished, would recognize that of the infinity of other truths he understands nothing.

There is not a single effect in Nature, not even the least that exists, such that the most ingenious theorists can ever arrive at a complete understanding of it. This vain presumption of understanding everything can have no other basis than never understanding anything. For anyone who had experienced just once the perfect understanding of one single thing, and had truly tasted how knowledge is attained, would recognise that of the infinity of other truths he understands nothing.

They say to know oneself is to know all there is that is human. But of course no one can ever know himself. Nothing human is finally calculable; even to ourselves we are strange.

Every man who has thought, knows not only how little he knows, but how little every other human being knows, and how ignorant, after all, the world must be.

Men are like May-flies. The most of them see things only during the brief breath of a day. The ages of history and biology are unknown and unconsidered. No wonder the universe is misunderstood. The human mind is without longitude. Even a cannonball, photographed with an exposure of one-five-thousandth of a second, seems suspended in mid-air.

According to his opinion, man should only believe what he can grasp with his intellectual faculties, or perceive by his senses, or what he can accept on trustworthy authority. Beyond this nothing should be believed.

This vain presumption, of understanding everything, can have no other basis than never understanding anything. For anyone who had experienced just once the understanding of one single thing, thus truly tasting how knowledge is accomplished, would then recognize that of the infinity of other truths, he understands nothing.

And whenever any one informs us that he has found a man who knows all the arts, and all things else that anybody knows, and every single thing with a higher degree of accuracy than any other man –whoever tells us this, I think that we can only imagine him to be a simple creature who is likely to have been deceived by some wizard or actor whom he met, and whom he thought all-knowing, because he himself was unable to analyze the nature of knowledge and ignorance and imitation.

What do we know ... of the world and the universe about us? Our means of receiving impressions are absurdly few, and our notions of surrounding objects infinitely narrow. We see things only as we are constructed to see them, and can gain no idea of their absolute nature. With five feeble senses we pretend to comprehend the boundlessly complex cosmos, yet other beings with wider, stronger, or different range of senses might not only see very differently the things we see, but might see and study whole worlds of matter, energy, and life which lie close at hand yet can never be detected with the senses we have.

Human knowledge is by necessity incomplete. We cannot know in advance what we might be able to know and what might be essentially unknowable. But of one thing we can be sure: if we do not try to find things out, we shall never succeed.