Why use "Freud's original ideas and approach" but not "Freud's original ideas and approaches"?

Why not just use "Freud's original idea and approach"? Beauty comes from better balance, after all.

Context:

Freud went on to develop theories about the unconscious mind and the mechanism of repression, and established the field of verbal psychotherapy by creating psychoanalysis, a clinical method for treating psychopathology through dialogue between a patient (or "analysand") and a psychoanalyst. Though psychoanalysis has declined as a therapeutic practice, it has helped inspire the development of many other forms of psychotherapy, some diverging from Freud's original ideas and approach. Freud postulated the existence of libido (an energy with which mental process and structures are invested), developed therapeutic techniques such as the use of free association (in which patients report their thoughts without reservation and make no attempt to concentrate while doing so), discovered the transference (the process by which patients displace on to their analysts feelings based on their experience of earlier figures in their lives) and established its central role in the analytic process, and proposed that dreams help to preserve sleep by representing as fulfilled wishes that would otherwise awake the dreamer. He was also a prolific essayist, drawing on psychoanalysis to contribute to the interpretation and critique of culture.

More:
http://en.wikipedia.org/wiki/Sigmund_Freud