Similar to CLT, Tracey D. Terrell's Natural Approach (1970s) is based on Stephen Krashen's theories of second language acquisition (L2A). Krashen argues that there is a natural process by which we acquire second languages. All that is needed is adequate exposure to 'comprehensible input' in stress-free situations, and the L2 will be acquired in much the same way as children learn their first language. Krashen draws a distinction between conscious learning and subconscious acquisition.