It is unquestionably one of the last objections Descartes might have expected, that if ideas exist, external objects are unknowable. How indeed could he have foreseen such an objection? Did he not seek to establish through his metaphysics, in the Meditations, the certainty of the existence of bodies, as well as the reality of the scientific knowledge we claim to have of them? And this procedure had necessarily to presuppose the existence of ideas: first, in order to demonstrate the existence (...) of God, then, to demonstrate the existence of bodies and to establish the reality of our knowledge of those objects. Hence Descartes could not have imagined the above-mentioned objection. And to my knowledge he does not envisge it at any moment in his writings. In the general form in which I have stated it it would have astonished Spinoza, Arnauld, Malebranche and Leibniz as well. And it did not fail to surprise Locke. The objection was formulated in 1697 by John Sergeant in his work: Solid Philosophy Asserted. In this paper I wish to present that objection. (shrink)