In Buddhist psychology, we classify mental maladaptation into the so called three poisons of greed, hate and delusion. This is a rough and ready idea since in most cases there is some mixture and the cut off points are not that clear. Nonetheless, it can be a useful way of thinking.
Basic Life Tendencies
This is because it corresponds to a very basic, primitive level of life. If one thinks of a very simple creature - an amoeba, say - this little blob of life moves along and encounters things which it then has to respond to or not. Some things it envelops and absorbs - food - some things it shrinks from in a reaction of fear and some things it has a confused response to.
So, at the most primitive level, life has these three tendencies. We could say that they are what make life interesting for an amoeba and in so many millions of years we have not progressed very far. We just have a more sophisticated way of being the same.
Actually, the term delusion can have a broad and a narrow meaning. In a broad sense, all three poisons are forms of delusion, but in the narrow sense of delusion as distinct from greed and hate, mostly has to do with a confused sense of identity. It is more of less synonymous with conceit. Conceit may be agrandising - a deep belief that one is better than anybody else - or denigratory - that one is worse - or, commonly wild fluctuation between the two. At the core of this is a sense that self-worth matters. From a Buddhist point of view, the healthy position is not one in which a person experiences positive self-worth all the time, but one in which the person stops thinking about self because it does not matter so much.
I think you can see that these two forms - agrandising and denigratory - are themselves forms of greed and hate respectively, so we can say that there are only two poisons - greed and hate, but that delusion in the narrow and usual sense refers when these conditions are directed toward the self rather than toward the external world. Delusion involves more deception and secrecy. Thus, greed and hate are actually less serious conditions than delusion, at least in the sense of being less complicated, closer to the basic life force of amoebic existence, and more on the surface whereas delusion is more deeply hidden.