A desire cannot, in itself, be either rational or irrational. It may conflict with other desires, and therefore lead to unhappiness ; it may rouse opposition in others, and therefore be incapable of gratification. But it cannot be considered "irrational" merely because no reason can be given for feeling it. We may desire A because it is a means to B, but in the end, when we have done with mere means, we must come to something which we desire for no reason, but not on that account "irrationally.
Source: Religion and Science, 1935, chapt. 9
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