In detail: he proposes (using the chart on the right) that "M1" causes "M2" (these are mental events) and "P1" causes "P2" (these are physical events). "M1" has "P1" as its supervenience base, and "M2" has "P2" as its supervenience base. The only way for "M1" to cause "M2", is by causing its supervenience base "P2" (a case of mental-to-physical causation). If "P1" causes "P2", and "M1" causes "P2", then we have a case of causal overdetermination. Applying the principle of causal-exclusion, either "P1" or "M1" must be eliminated as a cause of "P2". Given the principle of the causal closure of the physical domain, "M1" is excluded.