Full Idea
The constructivist ontological thesis is that mathematics owes its truth to the activity of an actual or ideal subject. The epistemological thesis is that we can have a priori knowledge of this activity, and so recognise its limits.
Gist of Idea
Constructivism is ontological (that it is the work of an agent) and epistemological (knowable a priori)
Source
Philip Kitcher (The Nature of Mathematical Knowledge [1984], 06.5)
Book Reference
Kitcher,Philip: 'The Nature of Mathematical Knowledge' [OUP 1984], p.142
A Reaction
The mention of an 'ideal' is Kitcher's personal view. Kitcher embraces the first view, and rejects the second.