Materialism has come under fire for its inability to fully explain altruism. There seems to be no physical explanation for why an individual performs selfless deeds for someone else with no personal benefit. It does not explain why some people are more altruistic than others, or why the same person might in some cases behave in an altruistic way, and in others not.
The only explanation for altruism, critics argue, is the presence of unconscious, and unobservable thought. A soul, as Descartes would argue, or at the very least a “thinking thing”.
Theory-theory is often contrasted with simulation theory. Simulation theory argues that we anticipate behaviour in others and interpret other people's actions by employing mental processes to project our own mental states onto others.
If I see someone lost and crying, I employ my mental processes to simulate how I would feel in their situation (empathy) and use this simulation to predict their behaviour. In this way, despite never having been in a similar situation myself, I can still predict and interpret the behaviour of someone in that situation.
However, simulation theory depends on the existence of purely mental processes to simulate and project my mental states onto those around me. It is therefore incompatible with the materialist argument.