In empirical research, developmentalists attribute knowledge to children on the basis of performances on assessment tasks. Since knowledge is not itself observable, criteria are needed for reasonable decisions to be made about how to classify different intellectual states, such as false belief, empirically true belief, necessary knowledge. In Piagetian research, there are two distinct views about the selection of such ‘response criteria’. One view states that knowledge can be attributed on the basis of children's judgements‐alone (J‐view). A contrary view states that such attribution requires children to justify their judgements (JJ‐view). An objection to the J‐view is discussed by consideration of the two dominant philosophical accounts as to the nature of knowledge, namely the foundationalist and causal accounts. It is argued, firstly, that the foundationalist account requires the elimination of the J‐view with which it is incompatible through its failure to deal with cases of knowledge at all. It is argued, secondly, that the causal account places a limitation on the J‐view which fails to deal with cases of necessary knowledge. Since one of the main problems in Piagetian research is the construction of necessary knowledge, only the JJ‐view should be used when this problem is addressed. Developmental research which is based on the J‐view alone could not, without circularity, be concerned with the substantive issues which are outside i