cannot logically be refuted, but which must be rejected on account of its absolute sterility. Roughly speaking, these assumptions are two in number: (1) there exist a plurality of percipients—minds capable of knowing; (2) there exists something which is not perceiving mind—the object of knowledge.
7. To these assumptions every philosophy of science which desires to have any social value—to be valid for more than a single mind—must add a postulate, which in the existence of what is not mind (the object of knowledge) demands the existence of something which as a characteristic of the object of knowledge is common to all minds, whether as a characteristic directly perceived, or inferred. By the external world, or briefly by the World, we understand this characteristic of the object of knowledge, which (at least potentially) is the common property of all minds, and on the identity of which they all agree.
7.1. The agreement of the characteristics of the object of cognition, postulated by this definition of the external world, implies the independence of the external world from the individual percipient, and accordingly the invariance of the external world with respect to a given type of mind (if by this type of mind we mean one subject to similar laws of thought), but not the independence of the external world from mind in general. Variously constituted minds will, in all probability, construct various “external worlds.” A world independent of mind in general would be a world common to all possible types of mind; but as minds constructed diversely from ours are not only unknown to us, but even impossible for us to imagine in respect of all possibilities of their type, such an absolute external world is not only unknowable, but a concept devoid of content.
7.2. It is clear then that neither the name of “external world,” nor its definition, conceals any supposition as to the mode of existence of the object of cognition; even if I presuppose (and this is a matter of indifference for physics) that only percepts can be an object of immediate knowledge, that therefore the mind can know only its own states, it is a matter of indifference whether I conjecture that these states are the mind’s own creations, or that they are caused by some external entity, existing outside the mind and independent of it. Physics, as we have already said above, is concerned only with percepts, and not with the manner in which they enter the mind, or arise within the mind; and the view to which we give the preference in this matter is a matter of personal predilection; as far as physics is
