From the Problems of Philosophy by Bertrand Russell

We can never truly judge that something with which we are not acquainted exists. This is by no means a truism, but on the contrary a palpable falsehood. If I am acquainted with a thing which exists, my acquaintance gives me the knowledge that it exists. But it is not true that conversely, whenever I can know that a thing of a certain sort exists, I or someone else must be acquainted with the thing. And then to consider what knowledge of general principles, if any, has the same kind of certainty as our knowledge of the existence of our own experiences will be dealt with in the following chapters.