Many philosophers and medical scientists assume thatdisease categories or entities used to classify concrete cases ofdisease, are often defined by disease mechanisms or causalprocesses. Others suggest that diseases should always be definedin this manner. This paper discusses these standpoints criticallyand concludes that they are untenable, not only when `diseasemechanism' refers to an objective mechanism, but also when`mechanism' refers to a pragmatically demarcated part of thetotal ``objective'' causal structure of diseases. As an alternativeto principles that use the concept of disease mechanism oranalogous concepts, a pragmatic approach is suggested anddescribed. This approach has been suggested before, but inproblematic or inadequate versions. This paper proposes a versioncompiled of two ``pragmatic principles'' and shows that they aremuch more adequate than the principle of disease mechanism. Withreference to a case study of a still ongoing internationaldiscussion of various candidates for a classification system formalignant lymphomas, including REAL (Revised European–AmericanClassification of Lymphoid Neoplasms) in which the concept ofdisease mechanism or analogous concepts plays a very small part,it is shown just how pivotal these two pragmatic principles canbe in actual discussions of definitions of diseases. Finally, itis pointed out that with regard to modern philosophy of languageit may, at least in some cases, be problematic to distinguishbetween the two pragmatic principles as they stand.