What Althusser does… is to rethink the concept of ideology in terms of Lacan’s ‘imaginary’. For the relation of an individual subject to society as a whole in Althusser’s theory is rather like the relation of the small child to his or her mirror-image in Lacan’s. In both cases, the human subject is supplied with a satisfyingly unified image of selfhood by identifying with an object which reflects this image back to it in a closed, narcissistic circle. In both cases, too, this image involves a misrecognition, since it idealizes the subject’s real situation. The child is not actually as integrated as its image in the mirror suggests; I am not actually the coherent, autonomous, self generating subject I know myself to be in the ideological sphere, but the ‘decentred’ function of several social determinants. Duly enthralled by the image of myself I receive, I subject myself to it; and it is through this ‘subjection’ that I become a subject.
There must be other leaps in life - as momentous as the "mirror stage" - that Lacan didn't mention. Some are universal; others, culturally particular. To understand that your parents are human (and not an element of the natural world), that they're separate from you, that they were children once, that they were born and came into the world, is another leap. It's as if you hadn't seen who they were earlier - just as, before you were ten months old, you didn't know it was you in the mirror. This happens when you're sixteen or seventeen. Not long after - maybe a year - you find out your parents will die. It's not as if you haven't encountered death already. But, before now, your precocious mind can't accommodate your parents' death except as an academic nicety - to be dismissed gently as too literary and sentimental. After that day, your parents' dying suddenly becomes simple. It grows clear that you're alone and always have been, though certain convergences start to look miraculous - for instance, between your father, mother, and yourself. Though your parents don't die immediately - what you've had is a realisation, not a premonition - you'll carry around this knowledge for their remaining decades or years. You won't think, looking at them, "You're going to die". It'll be an unspoken fact of existence. Nothing about them will surprise you anymore.