For in the first case bodies need only be thought through ontological predicates (pure concepts of understanding), eg as substance, to enable the proposition to be cognized a priori; whereas, in the second case, the empirical concept of a body (as a movable thing in space) must be introduced to support the proposition, although, once this is done, it may be seen quite a priori that the latter predicate (movement only by means of an external cause) applies to bodies.