3. In the last quarter of the last century, Slobin went back to Relativists' Sapir-Whorf Hypotheses to develop his 'thinking for speaking' thesis. He considers that “experiences are filtered though language into verbalized events” (1996: 75), that is to say, that language is not a sharper but a filter. Therefore, all the devices that speakers use to encode within linguistic terms their experiences constitute 'thinking for speaking'. This idea entails that, if people learn particular ways for 'thinking for speaking' in their native language, to learn a foreign language does not implies to have a good deal of knowledge of its grammar rules and a rich vocabulary but, it also involves to learn an appropriate 'thinking for speaking' in that target language.
4. While dealing with transfer, we cannot avoid to comment the 'Multiple Effects Principle' which gave place to two interesting concepts developed by Andersen and Kellerman in the last two decades of the last century. Without any kind of doubt, transfer is an important phenomenon to take into account in SLA and Andersen regards it as a filter “that governs the learner's perception and retention of specific features of the second language (L2) input” (1983: 117). He considers that the natural processes that a student of a target language acquires, together with the perception of the target language's structural relations with the source language, serves as a “catalyst for transfer to operate” (1983: 178). This phenomenon gave place to what he called 'Transfer to Somewhere Principle', so to say: A grammatical form or structure will occur consistently and to a significant extent in interlanguage as a result of transfer if and only if (1) natural acquisitional principles are consistent with the L1 structure or (2) there already exists within the L2 input the potential for (mis-) generalization from the input to produce the same form or structure. (1983: 182)