If the object of which knowledge is acquired is not given directly it has to be known through some other object, i.e. indirectly. Since the object is a state of affairs, and this itself is expressed in a sentence, every indirect acquisition of knowledge proceeds by inference from one sentence to another or by the derivation of the second sentence from the first. It is one of the most important insights of exact methodology that the truth of a sentence must be either apprehended directly or inferred; there is not, and furthermore there cannot be, any other way. In what follows, however, we shall be speaking, as is customary today, not of sentences but of (meaningful) statements.