 For Brouwer, pure mathematics consists primarily in the act of making certain mental constructions (Brouwer 1907: 99n.1 [1975:  61n.1]).[3]  The point of departure for these constructions is the intuition of the flow of  time.[4]  This intuition, when divested from all sensuous content, allows us to perceive the form “one thing and again a thing, and a continuum in between”. Brouwer calls this form, which unites the discrete and the continuous, “the empty two-ity”. It is the basic intuition of mathematics; the discrete cannot be reduced to the continuous, nor the continuous to the discrete (Brouwer 1907: 8 [1975: 17]).