How are we to define ‘red’? We seem to face a dilemma. For it seems that we must define ‘red’ in terms of ‘looks red’. But ‘looks red’ is semantically complex. We must therefore define ‘looks red’ in terms of ‘red’. Can we avoid this dilemma? Christopher Peacocke thinks we can. He claims that we can define the concept of being red in terms of the concept of being red′; the concept of a sensational property of visual experience. Peacocke agrees that his definition of red makes use of a concept that those who possess the concept of being red need not possess; namely, red′. But he thinks that this does not matter. For, he says, the definition is justified provided we can specify what it is to possess the concept of being red in terms of the concept of being red′. What he tries to show is that this might be so even if no-one could possess the concept of being red′ unless he possessed the concept of being red. Peacocke has two attempts at showing this. However, both these attempts fail. What Peacocke does show is something weaker. He shows that, using red′, we can construct a concept that gives what he calls the ‘constitutive role’ of the concept of being red; but, importantly, that it gives the constitutive role of red does not suffice for what Peacocke says is required for giving a definition. Thus, if we accept Peacocke's standard for definition, it follows that he gives us no way of avoiding the original dilemma. If this is right then perhaps we should join with those like Colin McGinn who think that we should give up our attempts to define our secondary quality concepts.