My own minor contribution here:
From The Input Hypothesis, Stephen D. Krashen, Longman, 1985
We progress along the natural order (hypothesis 2) by understanding input that contains structures at our next ’stage’ – structures that are a bit beyond our current level of competence (We move from i, our current level, to i + 1, the next level along the natural order, by understanding input containing i + 1…….
These are only hypotheses (5 of them) and have been much criticized in a scientific way, which is what you are supposed to do with hypotheses. That’s how science progresses. Criticize away on the statements as made. How do we determine i + 1? How do we provide it? How do we test for outcome? How do we deal with more than one learner, like in a class? Etc. Some criticisms fall into the theoretical realm and some fall into the applied realm, our balliwick.
And don’t forget, these hypotheses had a history in 1985 and there’s been tons of research done since then by both Krashen and about 13,435 other SLA researchers. It’s not a simple topic. It deals with how the human brain works, so how could it be simple?