Mathematics is the constructive activity of the human mind; mathematical statements are true only when we have explicit constructions (proofs) for them.
In Mathematical Intuitionism
Mathematical objects “exist” only as mental constructions or processes of construction; nonconstructive existence claims are rejected or reinterpreted.
The Problem with LEM for Intuitionism
Weak Indirect Proof
Strong Indirect Proof
The big elephant in the room
Platonism
Mathematical Platonism
Mathematics is the discovery of an objective realm of abstract mathematical entities (numbers, sets, functions) and their relations; mathematical practice uncovers truths about that realm.
In Mathematical Platonism
Mathematical objects exist independently of minds and language — they “live” in a timeless abstract realm; proofs reveal (rather than create) facts about these objects.