 An argument may be deductive without being formal. Although the paradigm cases of deduction do tend to occur in highly formalized systems, this is not necessary. “All even numbers greater than 2 are composite; 1058 is greater than 2; 1058 is even; hence 1058 is composite” is a perfectly good deduction, despite not being formalized. Hence, contrary to what is sometimes assumed in discussions of these issues, it is not true that all informal aspects of mathematical practice are thereby non-deductive.