MacWhinney's stimulating discussion suggests that there are many lines of argument that may address concerns raised by theorists who are concerned that there is a logical problem of language acquisition. This commentary argues: (1) that if the ‘logical problem’ applied to language, it would apply, with curious consequences to any learning by experience; (2) that the logical problem does not apply – given sufficient positive data from any reasonable language, language can be learned, in a probabilistic sense, by an ‘ideal learner’ using a simplicity principle; and (3) that a simplicity, or minimum description length, principle may provide a useful methodology for assessing claims concerning learnability of particular linguistic structures.