Some statements describe states of affairs that can never occur. An example is the statement ‘2+2=5’, assuming the terms ‘2’, ‘+’, ‘=’, and ‘5’ retain their standard meaning. Another example would ‘John owns three dogs, but no canines’.
Other statements describe states of affairs that can occur, but can never be stated truly. The medieval logician John Buridan gives ‘no statement is negative’ as an example. Other examples include ‘I am not here’ and ‘I am not speaking’.
In other words, there is a difference between something being impossible on account of what it is and its being impossible because its existence or obtaining would entail a contradiction.
Therefore, there is a difference between essence and existence.