… the formal homology (as well as substantial difference) between this reflexive logic of the Master-Signifier — the signifier of the lack of the signifier, the signifier which functions as a stand-in (filler) of a lack — and the logic of the objet petit a which is also repeatedly defined by Lacan as the filler of a lack: an object whose status is purely virtual, with no positive consistency of its own, only a positivization of a lack in the symbolic order. Something escapes the symbolic order, and this X is positivized as the objet a, the je ne sais quoi which makes me desire a certain thing or person.
However, this formal parallel between the Master-Signifier and the objet petit a should not deceive us: although, in both cases, we seem to be dealing with an entity which fills in the lack, what differentiates the objet a from the Master-Signifier is that, in the case of the former, the lack is redoubled, that is, the objet a is the result of the overlapping of the two lacks, the lack in the Other (the symbolic order) and the lack in the object — in the visual field, say, the objet a is what we cannot see, our blind spot in relation to the picture.
Each of the two lacks can operate independently of the other: we can have the lack of the signifier, as when we have a rich experience for which “words are missing” or we can have the lack in the visible for which, precisely, there is a signifier, namely the Master-Signifier, the mysterious signifier which seems to recapture the invisible dimension of the object.
Therein resides the illusion of the Master-Signifier: it coalesces with the objet a, so that it appears that the subject’s Other/Master possesses what the subject lacks. This is what Lacan calls alienation: the confrontation of the subject with a figure of the Other possessing what the subject lacks. In separation, which follows alienation, the objet a is separated also from the Other, from the Master-Signifier; that is, the subject discovers that the Other also does not have what he is lacking. The axiom Lacan follows is “no I without a”: wherever an I (unary feature, signifying mark that represents the subject) emerges, it is followed by an a, the stand-in for what was lost in the signification of the real.
Is, then, the objet a the signified of the S1 of the Master-Signifier? It may appear so, since the Master-Signifier signifies precisely that imponderable X which eludes the series of positive properties signified by the chain of “ordinary” signifiers (S2).
But, upon a closer look, we see that the relationship is exactly the inverse: with regard to the division between signifier and signified, the objet a is on the side of the signifier, it fills in the lack in/of the signifier, while the Master-Signifier is the “quilting point” between the signifier and the signified, the point at which the signifier falls into the signified. 598-599