Constructive processes performed with pencil and paper, the stricter view of the intuitionists, led by Brouwer, is that mathematics takes place primarily in the mind, and that written mathematics is secondary. One consequence of that is that Brouwer regards all axiomatizations of intuitionistic logic to be incomplete. Reflection can always uncover further intuitively true axioms of intuitionistic logic, and so it can never be regarded as being in final from.
Intuitionism represents the most fully formulated constructivist philosophy of mathematics. Two separable claims of intuitionism can be distinguished, which Dummett terms the positive and the negative theses.
The positive one is to the effect that the intuitionistic way of construing mathematical notions and logical operations is a coherent and legitimate one, that intuitionistic mathematics forms an intelligible body of theory. The negative thesis is to the effect that the classical way of construing mathematical notions and logical operations is incoherent and illegitimate, that classical mathematics, while containing, in distorted form, much of value, is, nevertheless, as it stands unintelligible.