I do not in fact entertain any doubts that physics is true in its main lines. The interpretation of physical formulæ is a matter as to which a considerable degree of uncertainty is possible; but we cannot well doubt that there is an interpretation which is true roughly and in the main. I shall come to the question of interpretation later; for the present, I shall assume that we may accept physics in its broad outlines, without troubling to consider how it is to be interpreted. On this basis, the above remarks on perception seem undeniable. We are often misled as to what is happening, either by peculiarities of the medium between the object and our bodies, or by unusual states of our bodies, or by a temporary or permanent abnormality in the brain. But in all these cases _something_ is really happening, as to which, if we turn our attention to it, we can obtain knowledge that is not misleading. At one time when, owing to illness, I had been taking a great deal of quinine, I became hypersensitive to noise, so that when the nurse rustled the newspaper I thought she was spilling a scuttle of coals on the floor. The interpretation was mistaken, but it was quite true that I heard a loud noise. It is a commonplace that a man whose leg has been amputated can still feel pains in it; here again, he does really feel the pains, and is only mistaken in his belief that they come from his leg. A percept is an observable event, but its interpretation as knowledge of this or that event in the physical world is liable to be mistaken, for reasons which physics and physiology can make fairly clear.