Attention may now be drawn to a general difficulty which may be raised with respect to the use of the terms 'looks' and 'appears'. It may be stated thus: 'If the lines are not convergent, how is it possible even to say that they _look_ convergent? Must it not be implied that at least under _certain_ circumstances we should perceive the lines as they are? Otherwise, why should we use the words 'look' or 'appear' at all? Moreover, this implication can be pushed further; for if we maintain that we perceive the real lines, we may reasonably be asked whether we must not under _all_ circumstances perceive them as they are. It seems as though a reality cannot be perceived except as it is.' It is the view to which this difficulty gives rise which is mainly responsible for the doctrine that the object of perception is not the reality, but an appearance. Since we do distinguish between what things look and what they are, it would seem that the object of perception cannot be the thing, but only an appearance produced by it. Moreover, the doctrine gains in plausibility from the existence of certain illusions in the case of which the reality to which the illusion relates seems non-existent. For instance, if we look steadily at the flame of a candle, and then press one eyeball with a finger, we see, as we say, two candles;[16] but since _ex hypothesi_ there is only one candle, it seems that what we see must be, not the candle, but two images or appearances produced by it. [16] Cf. Dr. Stout, on 'Things and Sensations' (_Proceedings of the British Academy_, vol. ii).