One definition of being would be that which distinguishes the actual from the merely possible. This is not a modal distinction within being; in thinking of it in this way you miss the peculiar privilege incurred by admission into the irreversible order of actuality, the family of being. Thought and imagination have modes, being does not, it is gentle, almost weightless, full of pathos and yet unforgiving, eternal. Our habits of thought, our very logic, make this hard to appreciate, and yet it is openness itself.