What characterizes a normal subject is precisely that he never takes seriously certain realities that he recognizes exist. You are surrounded by all sorts of realities about which you are in doubt, some of which are particularly threatening, but you don't take them fully seriously, for you think ... that the worst is not always certain, and maintain yourselves in an average, basic--in the sense of relation to the base--state of blissful uncertainty, which makes possible for you a sufficiently relaxed existence. Surely, certainty is the rarest of things for the normal subject. If he questions himself about this matter, he will be aware that certainty emerges in strict correlation to an action he undertakes.
His account of psychosis in terms of the foreclosure of the Name-of-the-Father is well known. But, to me it is less persuasive in light of his later work which recognizes the role of objet a in smaller, looser, more fragile chains of signification (an account which Zizek extends in his discussion of the decline of symbolic efficiency). Are there, then, other elements of Lacan's discussion of psychosis that might be useful? I think so.