But while each function is limited by its own separate line of objects, while the function of the eye serves only for the perception of the visible, the hand for the tangible, while walking finds an object in the space it crosses, thought, on the other hand, has everything for its object. Everything may be the object of understanding. Thought is not limited to any special object. Every phenomenon may be the object and the content of thought. More than this, we can only perceive anything when it becomes the object of our brain activity. Everything is therefore the object and content of thought. The faculty of thought may be exerted quite generally on all objects. We said a moment ago that everything may be perceived, but we now modify this to the effect that only perceivable things may be perceived. Only the knowable can be the object of knowledge, only the thinkable the object of thought. To this extent the faculty of thought is limited, for it cannot replace reading, hearing, feeling, and all other innumerable activities of the world of sensations. We do, indeed, perceive all objects, but no object may be exhaustively perceived, known, or understood. In other words, the objects are not wholly dissolved in the understanding. Seeing requires something that is visible, something which is, therefore, more than seeing. In the same way, hearing requires something that can be heard, thinking an object that can be thought of, something which is more than our thoughts, something still outside of our consciousness. We shall learn later on how we arrive at the knowledge that we see, hear, feel, and think of objects, and not merely of subjective impressions. By means of thought we become aware of all things in a twofold manner, viz., outside in reality and inside in thought, in conception. It is easy to demonstrate that the things outside are different from the things in our thoughts. In their actual form, in their real dimensions, they cannot enter into our heads. Our brain does not assimilate the things themselves, but only their images, their general outlines. The imagined tree is only a general object. The real tree is different from any other. And though I may have a picture of some special tree in my head, yet the real tree is still as different from its conception as the special is different from the general. The infinite variety of things, the innumerable wealth of their properties, has no room in our heads.