Before revision: Bertrand Russell among others believed the problem is Berkeley fails to differentiate between the act of perception and the content of it : "If we say that the things known must be in the mind, we are either un-duly limiting the mind's power of knowing, or we are uttering a mere tautology.
After revision: Bertrand Russell among others believed the problem is Berkeley 's argument "seems to depend for its plausibility upon confusing the thing apprehended with the act of apprehension"Bertrand Russell, The Problems of Philosophy, pages 22 to 23.</blockquote> : "If we say that the things known must be in the mind, we are either un-duly limiting the mind's power of knowing, or we are uttering a mere tautology.