f self-consciousness; and (3) that, since the manifold is
capable of entering into knowledge, and since we are capable of being
self-conscious, the categories, whose validity is implied by this
process, are valid.

Kant begins by pointing out[16] that all knowledge, _a priori_ as well
as empirical, requires the manifold, produced successively in the
mind, to be subjected to three operations.

[16] A. 95-104, Mah. 194-8.

1. Since the elements of the manifold are as given mere isolated
units, and since knowledge is the apprehension of a unity of connected
elements, the mind must first run through the multiplicity of sense
and then grasp it together into a whole, i. e. into an image.[17]
This act is an act of synthesis; it is called 'the synthesis of
apprehension' and is ascribed to the imagination. It must be carried
out as much in respect of the pure or _a priori_ elements of space and
time as in respect of the manifold of sensation, for individual spaces
and times contain a multiplicity which, to be apprehended, must be
combined.[18] The necessity of this act of synthesis is emphasized in
the second edition. "We cannot represent anything as combined in the
object without having previously combined it ourselves. Of all
representations, _combination_ is the only one which cannot be given
through objects,[19] but can be originated only by the subject itself
because it is an act of its own activity."[20]

[17] Cf. A. 120, Mah. 211.

[18] 'Combine' is used as the verb corresponding to
'synthesis'.

[19] I. e. given to us through the operation of things in
themselves upon our sensibility.

[20] B. 130, M. 80.

2. Since the data of perception are momentary, and pass away with
perception, the act of grasping them together requires that the mind
shall reproduce the past data in order to combine them with the
present datum. "It is plain that if I draw a line in thought, or wish
to think of the time from one midday to another, or even to represent
to myself a certain number, I must first necessarily grasp in thought
these manifold representations one after another. But if I were
continually to lose from my thoughts the preceding representations
(the first parts of the line, the preceding parts of time or the units
successively represented), and were not to reproduce them, while I
proceeded to the succeeding parts, there could never arise a complete
representation, nor any of the thoughts jus