And further, this source of error besets us in a more extended and more subtle manner than has yet been noticed. For our sympathetic apprehension of alien experiences of pleasure and pain has been so continually exercised, in so many ways, during the whole of our life, both by actual observation and oral communication with other human beings, and through books and other modes of symbolic suggestion; that it is impossible to say how far it has unconsciously blended with our own experience, so as to colour and modify it when represented in memory. Thus we may easily overlook the discrepancy between our own experience and that of others, in respect of the importance of certain sources of pleasure and pain, if no sudden and striking disappointment of expectations forces it on our notice. Only with considerable care and attention can sympathetic persons separate their own real likes and dislikes from those of their associates: and we can never tell whether this separation has been completely effected. But again: the practical inference from the past to the future is further complicated by the fact that we can alter ourselves. For it may be that our past experience has been greatly affected by our being not properly attuned to certain pleasures, as (_e.g._) those of art, or study, or muscular exercise, or society, or beneficent action; or not duly hardened against certain sources of pain, such as toil, or anxiety, or abstinence from luxuries: and there may be within our power some process of training or hardening ourselves which may profoundly modify our susceptibilities. And this consideration is especially important,--and at the same time especially difficult to deal with,--when we attempt to appropriate the experience of another. For we may find that he estimates highly pleasures which we not only have never experienced at all, but cannot possibly experience without a considerable alteration of our nature. For example, the pleasures of the religious life, the raptures of prayer and praise and the devotion of the soul to God, are commonly thought to require Conversion or complete change of nature before they can be experienced. And in the same way the sacrifice of sensual inclination to duty is disagreeable to the non-moral man when he at first attempts it, but affords to the truly virtuous man a deep and strong delight. And similarly almost all the more refined intellectual and emotional pleasures require training and culture in order to be enjoyed; and since this training does not always succeed in producing any considerable degree of susceptibility, it may always be a matter of doubt for one from whom it would require the sacrifice of other pleasures, whether such sacrifice is worth making.