 While it is clear that Locke envisioned some analogy between the real and nominal essences of substances with those of modes, it is unclear, however, just how that analogy should be cashed out. For example, Roger Woolhouse argues that it is a mistake for Locke to say that mode ideas have a real essence at all because there is nothing like a corpuscular structure that is analogous to the definition of a concept like triangle or lying. Moreover, there is, according to Woolhouse, nothing that binds the nominally essential qualities of modes together, whereas, for substances there is an underlying physical structure (see Woolhouse 1971, pp. 125–128). Indeed it is quite difficult to see how a mode could have a real essence if the model for what a real essence is comes from corpuscular structures. If Woolhouse is right, then Locke has no way to make good on his intended analogy and he is unable to adequately differentiate modes from substances.