It is most certainly true, as Professor Jodl says, that sensations do not depict the whole of reality. But why should they? Cognition is possible only by limiting the attention to a special point. Every sense organ is an organ of abstraction. Every sense depicts the effects of reality in its own way and in this way alone. It may freely be granted that there are many processes in reality which do not affect our sensibility. Yet there is nothing in reality which does not affect something in some way. If it did not, it could not be said to exist. The chemical rays of light do not affect our eye, they are invisible and were for that reason not noticed. But these rays are not without any effects. If we cannot observe them directly, we can invent sensitive plates or other instruments for observing their effects indirectly. Indirect observation makes it possible that the limitation of our senses does not result in a limitation of knowledge. Says Professor Jodl: “Our cognition of nature if we begin to construct always leads us to some _x_.” This sentence indicates that Professor Jodl’s and our conception of cognition are different. Cognition is not a reconstruction of assumed causes; it is a unification of our representative sensations or ideas. Something is again noticed, it is re-cognised, to be the same thing. Cognition is adaptation of new facts to our present stock of knowledge; it is the proper arrangement of new data in our system of mental representations. Cognition, accordingly, is the reduction of the unknown to terms of the known. How can it ever lead to an _x_? The positive conception of cognition is, as Kirchoff defines, it “an exhaustive and most simple description of facts.” It is a reconstruction of facts or, as Mach says, _Ein Nachbilden der Thatsachen_. Cognition is based upon _Anschauungen_; it will lead to an ultimate _x_, only in case we expect that cognition instead of being a description of facts will have to give us information about how it happens that facts exist, how they originated out of nothing. Professor Jodl’s thing in itself is not outside of Space and Time (as is Kant’s thing in itself) but it is the overwhelming infinitude of problems to be solved with which we cannot hope to get through even though our life lasted billions of light-years. Let me repeat here what I said in the second edition of “Fundamental Problems,”