First of all, you should realize that research is only one of several ways of "knowing."
The branch of philosophy that deals with this subject is called EPISTEMOLOGY. Epistemologists generally recognize at least four different sources of knowledge:
INTUITIVE KNOWLEDGE takes forms such as belief, faith, intuition, etc. It is based on feelings rather than hard, cold "facts."
AUTHORITATIVE KNOWLEDGE is based on information received from people, books, a supreme being, etc. Its strength depends on the strength of these sources.
LOGICAL KNOWLEDGE is arrived at by reasoning from "point A" (which is generally accepted) to "point B" (the new knowledge).
EMPIRICAL KNOWLEDGE is based on demonstrable, objective facts (which are determined through observation and/or experimentation).
Research often makes use of all four of these ways of knowing:
INTUITIVE (when coming up with an initial idea for research)
AUTHORITATIVE (when reviewing the professional literature)
LOGICAL (when reasoning from findings to conclusions)
EMPIRICAL (when engaging in procedures that lead to these findings)
Nevertheless, this last kind of knowledge, empirical knowledge, is what most modern research in TESL and language acquisition aims at establishing. That is why we call it empirical research.