According to the positivists, all our knowledge is based on experience which is the foundation not only of every empirical science, but also of those disciplines that are usually considered to be a priori. The paper consists of two main parts. Firstly, a positivist concept of number defended by J. S. Mill is presented; secondly, it is shown how this conception can settle some objections coming from apriori-oriented philosophers. Mill’s theory of number is interesting for at least two historical reasons. It is developed in connection with a relatively rich scholastic logic which is why its methodology is similar to the contemporary philosophy of language; it is indispensable for an appropriate comprehension of the concept of number that was proposed by Mill’s most famous opponent G. Frege.