The claim that human thoughts and choices are necessitated by outside forces is a positive supposition not supported by evidence, which should therefore be rejected by the burden-of-proof principle (pp. 1.3:72-3). (Behavioral experiments with pigeons or rats do not constitute evidence that processes of conceptual consciousness are determined by environmental factors.) In addition, the notion gives rise to epistemological difficulties similar to those encountered by AGNOSTIC (pp. 1.3:65-7) after he attempted to admit the possibility of alien mind control. If our thoughts are necessitated by outside forces, then we have no sound basis for accepting their validity or for any claim to knowledge.
This difficulty persists even if the determinist holds that the outside forces somehow impel our minds toward "correct" thoughts. For instance, a genetic determinist might argue that our minds are structured to recognize the truth automatically, because such recognition was a positive survival trait during the evolutionary process. Would this argument, if admitted, give us grounds to believe that our beliefs are true, or only that believing them to be true was a positive survival trait? Or only that the latter belief was a positive survival trait? If we accept this argument, then like AGNOSTIC, we are forced into an infinite regress, in which we are carried further and further from any possibility of logical conviction in determinism or any other belief, and we surrender any possibility of knowledge. 
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