My input on Krashen's 'Input Hypothesis' is that it really is the only way the brain learns language.
A language which is very much like your own will be swiftly learnt, but the same principles apply for languages completely dissimilar.
Raw, brute-forced input is the path to understanding language in the context of itself, without any forethought or reference to your native language.
The best evidence for input hypothesis I feel is the whole of human history prior to the Modern era.
William Adams, an Englishman alone in Japan, became a samurai in the early years of the 1600s.
He was completely fluent in Japanese and was a key advisor to the Shogun at the time.
He had no vocabulary lists or grammar books, nor did he have any bilingual companions, he had only his mind and an archipelago of native Japanese speakers.
This is a single anecdote, but vocabulary lists, complete grammars, and bilingual dictionaries of languages are wholly Modern inventions,
yet people have learned language quite naturally for tens of thousands (perhaps millions depending on when language was invented) of years.
It's only natural to understand nothing at first, but as long as you're paying attention to these things you can't understand, your brain is parsing them.
And after hundreds of hours of exposure (certainly less with closer related languages) your brain will naturally come to an understanding.
Additionally I should add that the perception that language is learned through intensive study, rather than extensive experience originates from the Modern industrialization of education.
The Industrial Western education system would not encourage something so creative and organic as learning a language through exposure, as the results of that are less concrete in the short-term.
It encourages rote memorization of grammar and vocabulary, and (in my experience) doesn't even acknowledge the necessity of immersion, nor is immersion in the target language encouraged.
It's goal is to make you the best, most rapid-fire translator of a language out there, able to hear Spanish and make it English, and turn your English back to Spanish on a whim.
But not to understand and think in Spanish on its own merits.
Four years of straight-'A' Spanish amount to 'literacy' without any actual fluency; no capacity to organically parse Spanish and formulate thoughts and responses in Spanish.
Sorry for the long, somewhat ranty post, but I felt it necessary to convey to yinz and any lurkers that Input Hypothesis IS the scientific fact (seriously look into Krashen, I believe he's proven that language can solely be learnt through input ultimately).
Learning with Modern tools such as Grammars and vocab lists CAN be useful, but it is in fact SECONDARY to pure linguistic input.
You must become FLUENT before you become LITERATE (though don't let that discourage you from working on both at the same time, just know the basis lies in fluency).
Post end now before