I will not again repeat that extension or magnitude having no known limits is a thinkable term, because the subject of thought is the quality of extension or magnitude; quantity not being essential to the idea of extension or magnitude. But I will pass to the idea of an existence which can not be defined. I suppose that by an existence is meant a being. If we undertake to think of a being whose quality we do not know to be the same as the quality of another being whom we do know, and the quantity of whose powers and faculties we can not measure, we propose, says Mr. Spencer, a term of impossible thought, because the law of thought can not be conformed to; the term can not be present to the mind, and no thinkable relation can be framed. Let this supposed difficulty be tested by a plain inquiry into that which we undertake to make the subject of thought when we think of a being who is said to be "unknowable." "Agnosticism" is a doctrine which eludes a definite grasp. I have seen it defined by one of its most distinguished professors in this way: "Agnosticism is of the essence of science, whether ancient or modern. It simply means that a man shall not say he knows or believes that which he has no scientific grounds for professing to know or believe.... Agnosticism simply says that we know nothing of what may be beyond phenomena."[97] Mankind are apt to be rather practical in their habits of thinking: experience teaches them that there is a well-founded distinction between knowledge and belief, when it comes to be a question of asserting the one or the other.[98] They find, too, by experience that, in regard to what they speak of when they say that they know a thing, there is a distinction to be observed in respect to the means of knowledge. No one hesitates to say that he knows there was such a man as Napoleon Bonaparte, although he never saw him, and although our knowledge of him is now derived from hearsay. But when we speak of knowing that a certain living person was at a certain spot on a certain day, we become immediately aware that in order to justify the assertion we or some one ought to have seen the person at the time and place, especially if anything important depends upon the assertion. There are a great many things that we say we know without scientific or other rigorous proof, and there are a great many other things which we do not say that we know without the kind of proof which is required. All our actions in life proceed upon this distinction, and we could not live in this world with any comfort if we did not act upon the assumption that we know things of which we have no scientific proof.