The causal chain appears in two forms: a mode of extension has its producing ground in a second mode of extension; a mode of thought can be caused only by another mode of thought--each individual thing is determined by one of its own kind. The two series proceed side by side, without a member of either ever being able to interfere in the other or to effect anything in it--a motion can never produce anything but other motions, an idea can result only in other ideas; the body can never determine the mind to an idea, nor the soul the body to a movement. Since, however, extension and thought are not two substances, but attributes of one substance, this apparently double causal nexus of two series proceeding in exact correspondence is, in reality, but a single one. (III. _prop_. 2, _schol_.) viewed from different sides. That which represents a chain of motions when seen from the side of extension, bears the aspect of a series of ideas from the side of thought. _Modus extensionis et idea illius modi una cademque est res, sed duobus modis expressa_ (II. _prop_. 7, _schol_.; cf. III. _prop_. 2, _schol_.). The soul is nothing but the idea of an actual body, body or motion nothing but the object or event in the sphere of extended actuality corresponding to an idea. No idea exists without something corporeal corresponding to it, no body, without at the same time existing as idea, or being conceived; in other words, everything is both body and spirit, all things are animated (II. _prop_. 13, _schol_.). Thus the famous proposition results; _Ordo et connexio idearum idem est ac ordo et connexio rerum (sive corporum; II. prop_. 7), and in application to man, "the order of the actions and passions of our body is simultaneous in nature with the order of the actions and passions of the mind" (III. _prop. 2, schol_.).