Hume, therefore, concludes, in this part of his work, that we cannot know of the existence of any "external object" whatever. And though in all that he says upon this subject, he is plainly thinking only of _material_ objects, the principles by which he tries to prove that we cannot know these must, I think, prove equally well that we cannot know any "external object" whatever--not even the existence of any other human mind. His argument is: We cannot directly observe any object whatever, except such as exist only when we observe them; we cannot, therefore, observe any "constant conjunctions" except between objects of this kind: and hence we can have no foundation in experience for any proposition which asserts the existence of any other kind of object, and cannot, therefore, know any such proposition to be true. And this argument must plainly apply to all the feelings, thoughts and perceptions of other men just as much as to material objects. I can never know that any perception of mine, or anything which I do observe, must have been caused by any other man, because I can never directly observe a "constant conjunction" between any other man's thoughts or feelings or intentions and anything which I directly observe: I cannot, therefore, know that any other man ever had any thoughts or feelings--or, in short, that any man beside myself ever existed. The view, therefore, which Hume suggests in this part of his work, flatly contradicts the view which he at first seemed to hold. He now says we _cannot_ know that a stone will fall, that fire will burn, or the sun will rise to-morrow. All that I can possibly know, according to his present principles, is that _I shall see_ a stone fall, shall feel the fire burn, shall see the sun rise to-morrow. I cannot even know that any other men will see these things; for I cannot know that any other men exist. For the same reason, I cannot know that Julius Caesar was murdered, or that all men are mortal. For these are propositions asserting "external" facts--facts which don't exist only at the moment when I observe them; and, according to his present doctrine, I cannot possibly know any such proposition to be true. No man, in short, can know any proposition about "matters of fact" to be true, except such as merely assert something about _his own_ states of mind, past, present or future--about these or about what _he himself_ has directly observed, is observing, or will observe. Here, therefore, we have a very different view suggested, as to the limits of human knowledge. And even this is not all. There is yet a third view, inconsistent with both of these, which Hume suggests in some parts of his work.