 In the philosophy of mathematics, constructivism asserts that it is necessary to find or construct a mathematical object to prove that it exists. In standard mathematics, one can prove the existence of a mathematical object without finding that object explicitly, by assuming its non-existence and then deriving a contradiction from that assumption. This proof by contradiction is not constructively valid. The constructive viewpoint involves a verificational interpretation of the existential quantifier, which is at odds with its classical interpretation. There are many forms of constructivism.These include the program of intuitionism founded by Browar, the finiteism of Hilbert and Bernice, the constructive recursive mathematics of Shannon and Markov, and Bishop's program of constructive analysis. Intuitionism also includes the study of constructive set theories such as CZF and the study of Toppe's theory. Constructivism is often identified with intuitionism, although intuitionism is only one constructivist program. Intuitionism maintains that the foundations of mathematics lie in the individual mathematician's intuition, thereby making mathematics into an intrinsically subjective activity.Two other forms of constructivism are not based on this viewpoint of intuition, and are compatible with an objective viewpoint on mathematics.