This introduces us to the relation of soul and body, or, more generally stated, to the relation of mind and matter. It is the theory of co-operation, of harmonious activity, which Leibniz substitutes for the theory which Descartes had formulated, according to which there are two opposed substances which can affect each other only through the medium of a _deus ex machina_. Locke, on the other hand, took the Cartesian principle for granted, and thus enveloped himself in all the difficulties which surround the question of "mind and matter." Locke wavers between two positions, one of which is that there are two unknown substances,--the soul and the object in itself,--which, coming in contact, produce sensations; while the other takes the hypothetical attitude that there may be but one substance,--matter,--and that God, out of the plenitude of his omnipotence, has given matter a capacity which does not naturally belong to it,--that of producing sensations. In either case, however, the final recourse is to the arbitrary power of God. There is no natural--that is, intrinsic and explicable--connection between the sensation and that which produces it. Sensation occupied the hard position which the mechanical school of to-day still allots it. It is that "inexplicable," "mysterious," "unaccountable" link between the domains of matter and mind of which no rational account can be given, but which is yet the source of all that we know about matter, and the basis of all that is real in the mind!