In the free discussion and criticism which has accompanied the progress of the work, we have enjoyed the constant advice and assistance of Professor and Mrs. Sidgwick, to each of whom we owe more than can be expressed by any conventional phrases of obligation. Whatever errors of judgment or ?aws in argument may remain, such blemishes are certainly fewer than they would have been but for this watchful and ever-ready help. Professor and Mrs. Sidgwick have also devoted some time and trouble, during vacations.