The Input Hypothesis and Affective Filter Hypothesis are untestable. The former contains vital constructs, of one. Which one?

i and i + 1, which Krashen himself has recognized are unoperationalizable, given the state of knowledge in interlanguage (IL) studies. The latter is a metaphor; to provide it with empirical content, Krashen would need to specify which affect variables, singly or in what combinations, and at what levels, serve to “raise the filter”.

Critical reviews of the teaching applications of MT have focused on the huge emphasis given by MT to grammar instruction, which has left little room for a pragmatic or usage approach to second language teaching. The idea that the teacher is not a mere communicator and thus has to interfere in order to provide the students with negative evidence has also been very much criticized.

The appeal to Chomskyan universal grammar (UG) to explain acquisition must mean that MT suffers from the same theoretical and empirical troubles that afflict other UG positions, issues and studies not mentioned at all by Krashen, who has yet to include in MT a “UG Hypothesis”.