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"][1] * ["The Use of Kripke's Schema as a Reduction Principle"][2] D. Van Dalen The Journal of Symbolic Logic Vol. 42, No. 2 (Jun., 1977), pp. 238-240 [1]: http://www.academia.edu/3528209/Constructive_recursive_functions_Church_thesis_and_Kripkes_schema [2]: http://www.jstor.org/stable/2272124