We have here the same general starting point of inquiry as we shall afterwards find in Kant. There are certain, obstinate puzzles which we meet with in discussion which can only be solved by going back and inquiring into the nature of knowledge and the powers of our minds. Unfortunately, as Kant points out, Locke went the wrong way about his task. He describes it as "a plain historical inquiry." He thought that he had only to look into his mind and see what was in it, as he might open a door and look into a room. The result is that he thinks of all knowledge as consisting simply in looking at what is present to the mind. We can know, therefore, whatever can be present to the mind, and the limitations of knowledge are discovered by asking what can be so present to the mind. The conclusions to which he comes as to different spheres of human inquiry are roughly these: We can have knowledge of mathematics because there we are concerned only with ideas present to the mind, and with noting their agreement and disagreement. We can have no knowledge of such questions as the immortality of the soul, or the nature of spirits, for they are beyond our observation. As regards existing things, we can have knowledge of them, in so far as they are present to our minds, and no further. The meaning of "present to the mind" was never clearly analysed by Locke; but he meant, for example, that we can observe that an object which is yellow, and which we call gold, is also heavy, and can be dissolved by Aqua Regia, but we cannot say why that is so, and we ought not, on Locke’s principles, to have any ground for supposing that these qualities will go on co-existing.