Because each species of organism has its own natural means of survival, each has different needs. A dog or a cat, for instance, needs to be able to smell its food—or it will refuse to eat and will die. Because humans maintain and enhance their lives by using their conceptual faculties (i. e., reason), our psychological health is particularly important and explains many of our most vital needs.

Within a species, objective needs may vary significantly from individual to individual. A woman may need more iron than a man; a gifted or learning-disabled child may have special educational needs. A person's specific needs may also depend on his or her state of knowledge. A young child may have great need for an elementary arithmetic text, while a well-educated adult has little need for it, except perhaps as an item for exchange. If the text is written in English, it may not serve the needs of a child who knows only Spanish.