The distinction between using a word, such as ‘cat’, to talk about cats, and mentioning the word, as one would if pointing out that ‘cat’ has three letters. The distinction becomes important when the possibility of mistaking use and mention arises, and philosophers often like to believe that those who do not share their views have made such a confusion. A belief shared by many logical positivists was that what appear to be perhaps fundamental or especially profound remarks about things (five is a number; necessarily matter is extended) are in fact reflections of conventions governing terminology (‘five’ refers to a number; ‘matter’ refers to whatever fills extended regions of space). See also formal/material mode of speech.