The field of transpersonal psychology has also been criticized for lacking conceptual, evidentiary, and scientific rigor. In a review of criticisms of the field, Cunningham writes, "philosophers have criticized transpersonal psychology because its metaphysics is naive and epistemology is undeveloped. Multiplicity of definitions and lack of operationalization of many of its concepts has led to a conceptual confusion about the nature of transpersonal psychology itself (i.e., the concept is used differently by different theorists and means different things to different people). Biologists have criticized transpersonal psychology for its lack of attention to biological foundations of behavior and experience. Physicists have criticized transpersonal psychology for inappropriately accommodating physic concepts as explanations of consciousness."