Recent studies demonstrated that 15-month-olds selectively respond to others’ beliefs. According to an epistemic mentalist interpretation, this attests that 15-month-olds possess a rudimentary capacity to attribute beliefs. Weaker interpretations suggest instead that infants are only sensitive to others’ beliefs because they can detect their proximal correlates. These two opposed interpretations often appeal to principled objections. In contrast, I argue that the dispute can be brought back to its empirical basis if we clearly explain what it means having ‘tacit’ or ‘implicit’ knowledge of a theory. I introduce a definition of tacit knowledge based on the notion of structural similarity, and I discuss some crucial experiments that may be conducted to assess the validity of the epistemic mentalist interpretation.