101. How, asked Berkeley, can an idea be like anything that is not an idea? Put the question in its proper strength--How can an idea be like that of which the sole and simple determination is just that it is not an idea (and such with Locke is body ‘in itself’ or as the real)--and it is clearly unanswerable. The process by which Locke was prevented from putting it to himself is not difficult to trace. ‘Body’ and ‘the solid’ are with him virtually convertible terms. Each indifferently holds the place of the substance, of which the primary qualities are so many determinations. [1] It is true that where solidity has to be defined, it is defined as an attribute of body, but conversely body itself is treated as a ‘texture of solid parts,’ _i.e._ as a mode of the solid. Body, in short, so soon as thought of, resolves itself into a relation of bodies, and the solid into a relation of solids, but Locke, by a shuffle of the two terms--representing body as a relation between solids and the solid as a relation between bodies--gains the appearance of explaining each in turn by relation to a simpler idea. Body, as the unknown, is revealed to us by the idea of solidity, which sense conveys to us; while solidity is explained by reference to the idea of body. The idea of solidity, we are told, is a simple idea which comes into the mind solely by the sense of touch. (Book II. chap. iii. sec. 1.) But no sooner has he thus identified it with an immediate feeling than, in disregard of his own doctrine, that ‘an idea which has no composition’ is undefinable (see Book III. chap. iv. sec. 7.), he converts it into a theory of the cause of that feeling. ‘It arises from the resistance which we find in body to the entrance of any other body into the place it possesses till it has left it;’ and he at once proceeds to treat it as the consciousness of such resistance. ‘Whether we move or rest, in what posture soever we are, we always feel something under us that supports us, and hinders our farther sinking downwards: and the bodies which we daily handle make us perceive that whilst they remain between them, they do by an insurmountable force hinder the approach of the parts of our hands that press them. That which then hinders the approach of two bodies, when they are moving one towards another, I call solidity.’ [2] [1] See Book II. chap. viii. sec. 23: The primary ‘qualities that are in bodies, are the bulk, figure, number, situation, and motion or rest, _of their solid parts_.’ Cf. Book II. chap. xiii. sec. 11: ‘Solidity is so inseparable an idea from body, that upon that depends its filling of space, its contact, impulse, and communication of motion upon impulse.’ [2] Book II. chap. iv. sec 7. In which he equivocates between body as unknown opposite of mind and body as a ‘nominal essence’.