  However, although mere differences in application do not undermine co-reference on Boyd’s account, other factors do. That is the point of departure of a criticism which Terrence Horgan and Mark Timmons have developed in a series of influential papers (first set out in Horgan and Timmons 1991 and 1992), in which they argue that Boyd’s causal approach also commits realists to implications of the type Hare pointed to.