the ego learns that it must inevitably go without immediate satisfaction, postpone gratification, learn to endure a degree of pain, and altogether renounce certain sources of pleasure. Thus trained, the ego becomes ‘reasonable,’ is no longer controlled by the pleasure-principle, but follows the REALITY-PRINCIPLE, which at bottom also seeks pleasure— although a delayed and diminished pleasure
Freud’s canonical formulation defines the reality principle as a modification of the pleasure principle whereby the ego accepts deferred and diminished gratification under the compulsion of necessity.
, Introductory Lectures on Psycho-Analysis, 1917thesis