To exist is to be something, as distinguished from the nothing of non-existence, it is to be an entity of a specific nature made of specific attributes.
The development of human cognition starts with the ability to perceive things, i.e., entities. Of man’s five cognitive senses, only two provide him with a direct awareness of entities: sight and touch. The other three senses — hearing, taste and smell — give him an awareness of some of an entity’s attributes (or of the consequences produced by an entity): they tell him that something makes sounds, or something tastes sweet, or something smells fresh; but in order to perceive this something, he needs sight and/or touch.
The concept “entity” is (implicitly) the start of man’s conceptual development and the building-block of his entire conceptual structure. It is by perceiving entities that man perceives the universe.
The first concepts man forms are concepts of entities — since entities are the only primary existents. (Attributes cannot exist by themselves, they are merely the characteristics of entities; motions are motions of entities; relationships are relationships among entities.)
This term [entity] may be used in several senses. If you speak in the primary sense, “entity” has to be defined ostensively — that is to say, by pointing. I can, however, give you three descriptive characteristics essential to the primary, philosophic use of the term, according to Objectivism. This is not a definition, because I’d have to rely ultimately on pointing to make these points clear, but it will give you certain criteria for the application of the term in the primary sense . . . .
1. An entity means a self-sufficient form of existence — as against a quality, an action, a relationship, etc., which are simply aspects of an entity that we separate out by specialized focus. An entity is a thing.
2. An entity, in the primary sense, is a solid thing with a definite boundary — as against a fluid, such as air. In the literal sense, air is not an entity. There are contexts, such as when the wind moves as one mass, when you can call it that, by analogy, but in the primary sense, fluids are not entities.
3. An entity is perceptual in scale, in size. In other words it is a “this” which you can point to and grasp by human perception. In an extended sense you can call molecules — or the universe as a whole — “entities,” because they are self-sufficient things. But in the primary sense when we say that entities are what is given in sense perception, we mean solid things which we can directly perceive.