The idea of comparing Whitehead with Bion was born when I first met Whitehead's idea, "the fallacy of misplaced concreteness" (FMC), and felt immediately that I knew it from my knowledge of Bion's work. The paper begins by indicating some of the elements in Alfred North Whitehead's "process philosophy." There is a brief survey of object relations theory, particularly Kleinian psychodynamics, as introduction to the aspect of Wilfred Bion's work which explored the nature of thought. The unconscious processes he named "truth" and "lie" in thinking are identified. Comparison between these two thinkers begins from common ground, the notions of process, fallacious or truthful, in thought, and looks at the differences of perspective each brought to the problems of methodology in thinking. It is shown that each accepts that "true" thought is open to query and interrogation, and that it is not possible to know whether or not a particular thought is "true," as conscious thought cannot access primary undistorted perception of an objective world. A claim is made that a particular conscious discipline in mental awareness, akin to the use of countertransference in psychodynamic thought, will however increase the probability that a thought process is "thinking truly" rather than thinking in a fallacious way.