According to Spinoza, “Love is nothing else but pleasure accompanied by the idea of an external cause”. Author proposes that desire is nothing else but a change of pleasure accompanied by the idea of its cause, that terms ‘desire', ‘want' and their cognates describe change of the pleasantness of the state of a subject (PSS in short) associated with X, that if change of PSS is positive/negative, then X is called desirable/undesirable correspondingly. Both positive and negative desires can be strong, so strength of desire characterizes its magnitude. Need of X is defined here as a cyclical desire of X that gets stronger/weaker with dissatisfaction/satisfaction of its need. Author also explores an idea that the stronger is desire of X by a subject, the more attention this subject pays to X. Distribution of attention and influence on it by the will effort are analyzed in this paper.