Let us assume that a certain percept, e.g., red, appears in consciousness. To continued observation, the percept shows itself to be connected with other percepts, e.g., a certain figure, temperature, and touch-qualities. This complex of percepts I call an object in the world of sense. I can now ask myself: Over and above the percepts just mentioned, what else is there in the section of space in which they are? I shall then find mechanical, chemical, and other processes in that section of space. I next go further and study the processes which take place between the object and my sense-organs. I shall find oscillations in an elastic medium, the character of which has not the least in common with the percepts from which I started. I get the same result if I trace further the connection between sense-organs and brain. In each of these inquiries I gather new percepts, but the connecting thread which binds all these spatially and temporally separated percepts into one whole, is thought. The air vibrations which carry sound are given to me as percepts just like the sound. Thought alone links all these percepts one to the other and exhibits them in their reciprocal relations. We have no right to say that over and above our immediate percepts there is anything except the ideal nexus of precepts (which thought has to reveal). The relation of the object perceived to the perceiving subject, which relation transcends the bare percept, is therefore purely ideal, i.e., capable of being expressed only through concepts. Only if it were possible to perceive how the object of perception affects the perceiving subject, or, alternatively, only if I could watch the construction of the perceptual complex through the subject, could we speak as modern Physiology, and the Critical Idealism which is based on it, speak. Their theory confuses an ideal relation (that of the object to the subject) with a process of which we could speak only if it were possible to perceive it. The proposition, "No colour without a colour-sensing eye," cannot be taken to mean that the eye produces the colour, but only that an ideal relation, recognisable by thought, subsists between the percept "colour" and the percept "eye." To empirical science belongs the task of ascertaining how the properties of the eye and those of the colours are related to one another; by means of what structures the organ of sight makes possible the perception of colours, etc. I can trace how one percept succeeds another and how one is related to others in space, and I can formulate these relations in conceptual terms, but I can never perceive how a percept originates out of the non-perceptible. All attempts to seek any relations between percepts other than conceptual relations must of necessity fail.