At the top of the figure are the subjects which begin the transition to advanced mathematical thinking. All of these require significant cognitive reconstructions. Euclidean proof requires the realisation of the need of systematic organisation, and
agreed ways of verbal deduction for visually inspired proof (the use of congruent triangles). The move into calculus has the difficulties caused by the limit procept. The move into more advanced algebra (such as vectors in three and higher dimensions) involves such things as the vector product which violates the commutative law of multiplication, or the idea of four or more dimensions, which overstretches and even severs the visual link between equations and imaginable geometry.