The argument of § 3 runs as follows: "Geometry is a science which determines the properties of space synthetically and yet _a priori_. What, then, must be the representation of space, in order that such a knowledge of it may be possible? It must be originally perception, for from a mere conception no propositions can be deduced which go beyond the conception, and yet this happens in geometry. But this perception must be _a priori_, i. e. it must occur in us before all sense-perception of an object, and therefore must be pure, not empirical perception. For geometrical propositions are always apodeictic, i. e. bound up with the consciousness of their necessity (e. g. space has only three dimensions), and such propositions cannot be empirical judgements nor conclusions from them." "Now how can there exist in the mind an external perception[28] which precedes[29] the objects themselves, and in which the conception of them can be determined _a priori_? Obviously not otherwise than in so far as it has its seat in the subject only, as the formal nature of the subject to be affected by objects and thereby to obtain _immediate representation_, i. e. _perception_ of them, and consequently only as the form of the external sense in general."[30] [28] 'External perception' can only mean perception of what is spatial. [29] _Vorhergeht._ [30] 'Formal nature _to be affected by objects_' is not relevant to the context. Here three steps are taken. From the _synthetic_ character of geometrical judgements it is concluded that space is not something which we _conceive_, but something which we _perceive_. From their _a priori_ character, i. e. from the consciousness of necessity involved, it is concluded that the perception of space must be _a priori_ in a new sense, that of taking place _before_ the perception of objects in it.[31] From the fact that we perceive space before we perceive objects in it, and thereby are able to anticipate the spatial relations which condition these objects, it is concluded that space is only a characteristic of our perceiving nature, and consequently that space is a property not of things in themselves, but only of things as perceived by us.[32] [31] Cf. B. 42, M. 26 (a) fin., (b) second sentence. [32] Cf. B. 43, M. 26-7.