Why it is difficult to study

Like dreams, déjà experiences are eminently subjective and cannot be evoked at will. They arise sporadically, sometimes more frequently and sometimes less so. Some are more intense than others. Some happen very rapidly while others last for longer times. Up till now, as far as I know, all efforts at evoking them, either with psychological methods or with neurological probes, have not produced consistent, reproducible results. This impels us, at the present time, to adopt a phenomenological approach in which we gather what data we can and see if we can discover regularities. Maybe later we shall have a better idea of what function such experiences serve and what mechanisms are involved, but at present we are grateful for accounts and all the data we can accumulate.