The term “Multiple Intelligences” was first coined by Harvard psychologist Howard Gardner. His theory is spelled out in the 1983 book, Frames of Mind: The Theory of Multiple Intelligences. In the book, Gardner posits that humans possess many varied types of intelligence, not just one. This stands in stark contrast to IQ and standardized testing, both of which look at intelligence as a one-dimensional concept: you either have it or you don’t. While Gardners’s work is still somewhat controversial, I think it is a helpful way to frame intelligence and useful tool for choosing effective language learning methods and materials for oneself.
Michael Heim (1943-2012) was a highly respected professor of Slavic languages at UCLA. He spoke 10 languages (though he claimed that the number is hard to tie down due to the slippery political nature of language-dialect distinctions). Heim believed that effective language learning must begin (and progress) with a specific purpose in mind; he never learned a language just for the heck of it.