If a traditional foundationalist believes that beliefs are justified by sense-experience, he’s a justificationalist. The argument in the OP works. How can he justify the belief that beliefs are justified by sense-experience without first assuming his conclusion?
If he believes belief are only justified by experience, that could a problem. Otherwise, he could use reductio, analysis, abduction, all sort of things.
If he believes belief are only justified by experience, that could a problem. Otherwise, he could use reductio, analysis, abduction, all sort of things.