ost rigorously determined; every individual, finite,
determinate thing and event is determined to its existence and action by
another similarly finite and determinate thing or event, and this cause is,
in turn, determined in its existence and action by a further finite mode,
and so on to infinity (I. _prop_. 28). Because of this endlessness in the
series there is no first or ultimate cause in the phenomenal world; all
finite causes are second causes; the primary cause lies within the sphere
of the infinite and is God himself. The modes are all subject to the
constraint of an unbroken and endless nexus of efficient causes, which
leaves room neither for chance, nor choice, nor ends. Nothing can be or
happen otherwise than as it is and happens (I. _prop_. 29, 33).

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 ac