But we are persistently told: there are in reality two ways of arriving at the cognition of objects--from within and from without. These two ways are as opposite as the right and wrong side of a stuff. It is in this sense that psychology is the science of the within and looks at the wrong side, while the natural sciences reckon, weigh, and measure the right side. And this is so true, they add, that the same phenomenon absolutely appears under two forms radically different from each other according as they are looked at from one or the other of the two points of view. Every one of our thoughts, they point out to us, is in correlation with a particular state of our cerebral matter; our thought is the subjective and mental face, the corresponding cerebral process is the objective and material face. Though this dualism is frequently presented as an observed truth, I think it is possible to show its error. Take an example: I look at the plain before me, and see a flock of sheep pass through it. At the same time an observer, armed with a microscope _a la_ Jules Verne, looks into my brain and observes there a certain molecular dance which accompanies my visual perception. Thus, on the one hand, is my representation; on the other, a dynamic state of the nerve cells. This is what constitutes the right and the wrong sides of the stuff. We are told, "See how little resemblance there is in this; a representation is a psychical, and a movement of molecules a material, thing." But I, on the contrary, think there is a great resemblance. When I see the flock passing, I have a visual perception. The observer who, by the hypothesis, is at that moment looking into my brain, also experiences a visual perception. Granted, they are not the same perception. How could they be the same? I am looking at the sheep, he is looking at the interior of my brain; it is not astonishing that, looking at objects so different, we should receive images also very different. But, notwithstanding their difference of object--that is, of content--there are here two visual perceptions composed in the same way; and I do not see by what right it can be said that one represents a material, the other a physical, phenomenon. In reality, each of these perceptions has a two-fold and psycho-physical value--physical in regard to the object to which it applies, and psychical inasmuch as it is an act of perception, that is to say, of consciousness. For one is just as much psychical as the other, and as much material, for a flock of sheep is as material a thing as is my brain. If we keep this conclusion in our minds, when we come to make a critical examination of certain philosophical systems, we shall easily see the mistake they make.