"We gain the notion of the independancy of objects, from the observation of one object affecting many minds in a manner which renders it impossible there should be as many objects as minds. If five men see a pond, and can only walk round one pond, then there is one pond seen five times over, not five ponds; so the pond whatever it may be when unperceived, must at least in its unperceived state, be independant of, and I may add external to all the minds; for if the pond were only in the mind, there would be five ponds, and every person who perceived a pond would create another pond, and yet this multitude of ponds in perception would in many respects but merit the definition due to one pond. Thsu there would be such a contradiction among the 'ideas and sensations,' that the mind must come to the belief of only one pond, seen by five persons; that is, in other words, an independant cause for particular sensations. This objection to his doctrine Berkeley answers, in a very unsatisfactory, hesitating manner in his dialogues."
Lady Mary Shepherd, Essays on the Perception of an External Universe, Part I ("An Essay on the Academical or Sceptical Philosophy as Applied by Mr. Hume to the Perception of External Existence"), pp. 80-81. ('Independant' with an 'a' is Shepherd's standard spelling.)