Another point is important. Reid, as I have said, had specially prided himself upon his supposed overthrow of Berkeley's idealism. He was considered to have shown, in spite of sceptics, that the common belief in an external world was reasonable. Brown in his lectures ridiculed Reid's claim. This 'mighty achievement,' the 'supposed overthrow of a great system,' was 'nothing more than the proof that certain phrases are metaphorical, which were intended by their authors to be understood _only_ as metaphors.'[480] The theory was dead before Reid slew it, though the phrases were still used as a mere 'relic,' or survival of an obsolete doctrine.[481] The impossibility of constructing extension out of our sensations is the _experimentum crucis_ upon which Reid was ready to stake his case. If the attempt at such a construction could succeed, he would 'lay his hand upon his mouth' and give up the argument.[482] Brown takes up the challenge thus thrown out. He holds that our knowledge of an external world is derived from a source which Reid overlooked. He modifies the Scottish psychology by introducing the muscular senses. His theory is that the infant which has learned to move discovers that on some occasions its movements are modified by a sense of 'impeded effort.'[483] The sudden interruption to a well-known series excites in its mind the notion of 'a cause which is not in itself.' This is the source of our belief in an external world. That belief is essentially the belief in some cause which we know to be other than our own mental constitution or the series of 'internal' phenomena, and of which we can know nothing else. It is enough to indicate a theory which has been elaborated by later psychologists, and plays a great part (for example) in the theories of Mill, Bain, and Mr. Herbert Spencer. It shows the real tendency of Brown's speculations. In the first place, it must be noticed that the theory itself had been already emphatically stated by Destutt de Tracy. Hamilton accuses Brown of plagiarism.[484] Whether his accusation be justifiable or not, it is certainly true that Brown had in some way reached the same principles which had been already set forth by a leading 'ideologist.' Brown, that is, though the official exponent of the Scottish philosophy, was in this philosophical tenet at one with the school which they regarded as materialistic or sceptical. The path by which he reaches his conclusions is also characteristic.