The term, due to Waismann, for the fact that however tightly we think we define an expression, there always remains a set of (possibly remote) possibilities under which there would be no right answer to the question of whether it applies. This is its porosity, or open texture. It is not the same as vagueness, since in actual situations the application of the term may be quite definite. For example, the term ‘mother’ is not vague, but its open texture is revealed if through technological advance differences open up between the mother that produces the ovum, the mother that carries the foetus to term, and the mother that rears the baby. It will then be fruitless to pursue the question of which is the ‘real’ mother, since the term is not adapted to giving a decision in the new circumstances.