PHIL2206 Philosophy of Mind
Week 5: The Identity Theory

Psychoneural (i.e. mind-brain) correlations

  1. We have found (from experiment) that there are correlations between types (or kinds, or properties) of mental events and types (or kinds, or properties) of physical brain events.
  2. We have found, for example, the following correlation between being in pain (a kind of mental event) and having one's c-fibres firing (a kind of physical brain event): whenever someone is in pain her c-fibres are firing, and whenever someone's c-fibres are firing she is in pain.
  3. In fact, we have evidence for the following:

    Mind-brain correlation thesis
    For each type, M, of mental event there exists a type, B, of physical brain event (called the neural correlate of M) such that for all x: x undergoes an event of type M iff x undergoes an event of type B.

  4. According to this, there exists a type, B, of physical brain event such that for all x: x is in pain iff x undergoes an event of type B. We have found one such B: having one's c-fibres firing. There may be other such B (i.e. other neural correlates of being in pain), but we shall assume there is just one (and so too for all other types of mental event).
  5. If the mind-brain correlation thesis is true, then it cries out for explanation: why are there such correlations?

Some possible explanations

    Why do events of type M correlate with events of type B?

  1. Causal interactionism (e.g. Descartes). Because events of type M cause events of type B (this explains one direction of the correlation), and events of type B cause events of type M (this explains the other direction).

    Analogy. Correlation: whenever the temperature falls below 20F (-6C) and stays there for several days, the lakes freeze. Explanation: the falling of the temperature causes the freezing of the lakes.

  2. Preestablished harmony (e.g. Leibniz). Because events of type M and events of type B were once synchronised and have since stayed that way.

    Analogy. Correlation: whenever clock 1 shows time t clock 2 shows time t, and vice-versa. Explanation: the clocks were once set to the same time (and both are working correctly).

  3. Occasionalism (e.g. Malebranche). Because whenever an event of type M occurs in someone, God causes an event of type B to occur in her, and vice-versa.

    Analogy. Correlation:

  4. The double-aspect theory (e.g. Spinoza). Because events of type M and events of type B are two aspects of a single event.

    Analogy. Correlation: for a fixed amount of gas in a rigid container, whenever the temperature of the gas changes the pressure of the gas changes, and vice-versa. Explanation: change in temperature and change in pressure are different aspects of change in speed of the gas molecules. Temperature and pressure are measures of the gas that both depend on the speed of the gas molecules.

  5. Epiphenomenalism (e.g. T. H. Huxley). Because events of type M are caused by and only by events of type B.

    Analogy. Correlation: whenever I move my shadow moves, and vice-versa. Explanation: my moving, and only my moving, causes my shadow to move.

  6. Emergentism. Because nothing: the correlations are brute facts - they cannot be explained.
  7. The identity theory. Because M is B. Being in pain, for example, is having one's c-fibres firing.

    Analogy. Correlation: whenever there is lightning there is atmospheric electrical discharge, and vice-versa. Explanation: lightening is atmospheric electrical discharge.

More about the identity theory

  1. It is not meant to be an a priori truth that being in pain is having one's c-fibres firing. Rather, it is something that we have discovered by a posteriori investigation.

    It is thus not like the claim all bachelors are male, which is knowable a priori: anyone who understands 'bachelor' is in a position to know that all bachelors are male. It is more like the claims that water is H2O, that heat is molecular kinetic energy, and that light is electromagnetic radiation.

  2. The identity theory is a type-type identity theory, rather than a token-token identity theory. The claim is that the type M is identical to the type B, not that each token of the type M is identical to a token of the type B, and vice-versa (i.e. that each token of the type A is also a token of the type B, and vice-versa).

    Compare the following two claims: (a) Every coloured thing is also a shaped thing (a token-token identity theory). (b) Being coloured is being shaped (a type-type identity theory).

  3. A type-type identity theory is much more interesting than a token-token identity theory. Just because every token of type M is also a token of type B, and vice-versa, there need not be any interesting correlations between type M and type B, so a token-token identity theory does not explain why there are such correlations.
  4. The type-type identity theory is also logically stronger than the token-token identity theory: the former entails but is not entailed by the latter.

Some arguments for the identity theory

  1. A parsimony argument.
  2. Two explanatory arguments.
  3. A causal argument.

Some arguments against the identity theory

  1. Two epistemological arguments.
  2. A modal argument.
  3. A multiple realization argument.