Harel‟s view, mathematics consists of two complementary subsets: (a) the first consists of
institutionalized ways of understanding, which is a collection of established definitions, axioms,
theorems, proofs, problems, and solutions that have been accepted by the mathematical
community; and (b) the second is a collection of ways of thinking, which are conceptual tools
that are useful for the generation of the first subset (Harel, 2008). The distinction between ways
of thinking and ways of understanding underscores the importance of mathematical habits of
mind, which tend to be neglected in traditional mathematics curricula.
According to Harel‟s duality principle (2007), “Students develop ways of thinking only
through the construction of ways of understanding, and the ways of understanding they produce
are determined by the ways of thinking they possess” (p. 272). This principle asserts that ways of
thinking cannot be improved independently of ways of understanding, and vice versa. Hence,
Harel advocates that both ways of understanding and ways of thinking should be incorporated as
learning objectives for students.