Is, rather than is not. The assertion that a thing is there to be reckoned with.
Latin existere/exsistere, ex- (out) plus sistere (to stand); the third-person assertion of being.
Its root is a standing-out, not a passive being: sistere meant to cause to stand, to plant something upright where it can be seen. What exists has, at root, been made to stand somewhere rather than merely happening to be.