You are probably referring to Brouwer's considerations of the Creative Subject, which can be formulated mathematically as Kripke's schema. It implies that all subsets of $\mathbb{N}$ are countable, for example. I am having trouble finding good references, maybe these two will get you started:
Göran Sundholm: "Constructive recursive functions, Church's thesis and Kripke's schema"
"The Use of Kripke's Schema as a Reduction Principle" D. Van Dalen The Journal of Symbolic Logic Vol. 42, No. 2 (Jun., 1977), pp. 238-240