Maslow's primary contribution to psychology is his Hierarchy of Human Needs, which he often presented as a pyramid. These needs are classified as "conative needs", "cognitive needs", and "aesthetic needs". "Neurotic needs" are included in Maslow's theory but do not exist within the hierarchy.
The needs at the bottom of the pyramid are needs that are the strongest. These are also needs most related to animals. The higher on the pyramid, the weaker the need is to the individual.
The first four layers of the pyramid are what Maslow called "deficiency needs" or "D-needs": the individual does not feel anything if they are met, but feels anxious if they are not met. Needs beyond the D-needs are "growth needs", "being values", or "B-needs". When fulfilled, they do not go away; rather, they do not motivate further.
The base of the pyramid is formed by the physiological needs, including the biological requirements for food, water, air, and sleep. Once these physiological needs are met, an individual can concentrate on the second level, the need for safety and security.
The third level is the need for love and belonging, followed by the need for status and esteem. Finally, self-actualization sits at the apex of the original pyramid.
In 1970 Maslow published a revision to his original 1954 pyramid[1], adding the cognitive needs (first the need to acquire knowledge, then the need to understand that knowledge) above the need for self-actualization, and the aesthetic needs (the needs for beauty, balance, structure, etc.) at the top of the pyramid. However, not all versions of Maslow's pyramid include the top two levels.
