Socrates is pale. Socrates is a man. Obviously, the former presupposes the latter. The infinitive 'to be' is restricted both by the subject and the predicate of the above statements. When we say of something that it exists, X exists, we cannot specify what 'exists' means until we know the value of the variable.
Existence is either substantial or accidental.
To be and that which is are different. For to be itself is not yet, but that which is, having received a form of being, is and subsists. Vivere est esse viventibus . For a living thing, to be is to live. Esse substantiale .