C.S. Lewis’ argument from reason against naturalistic atheism

By Atheist Answers (Facebook Page)

No matter how contentious an intellectual debate may appear, both parties agree on at least one thing.

They both assume that rationality, if properly used, leads to true conclusions.

The laws of identity, non-contradiction, and excluded middle, for example, accurately describe reality.

If human perceptions about these basic truths were incorrect, then it would be impossible to reason to any conclusion.

Theists argue that this necessary presupposition is incompatible with a naturalistic worldview. If naturalism is true then rationality is not reliable, undercutting all beliefs including acceptance of naturalism itself.

Arguments of this genre are coined “arguments from reason.”

C.S. Lewis’ Argument from Reason

C.S. Lewis advanced an argument from reason that can very generally be summarized as follows:

(1) If adherence to a worldview makes it impossible to believe that rational thinking is reliable, that worldview should be rejected.

(2) A naturalistic worldview makes it impossible to believe that rational thinking is reliable.

(3) Therefore, a naturalistic worldview should be rejected.

Review of Premises

Premise (1) is uncontroversial.

“A theory which explained everything else in the whole universe but which made it impossible to believe that our thinking was valid, would be utterly out of court. For that theory would itself have been reached by thinking, and if thinking is not valid that theory would, of course, be itself demolished.”

Premise (2) is the heart of Lewis’ argument which he supports with a sub-argument:

(2.1) Rational thinking is reliable only if it is based upon recognizing ground-consequent relationships.

(2.2) If two things are related by cause-effect, they are not also related by ground-consequent.

(2.3) According to naturalism, all phenomena are explained only on a cause-effect basis.

(2.4) The category of “all” phenomena includes rational thinking.

(2.5) Naturalism requires that rational thinking be based upon cause-effect relationships rather than ground-consequent relationships.

(2.6) Therefore, a naturalistic worldview makes it impossible to believe that rational thinking is reliable.

Lewis’ argument hinges on his distinction between cause-effect and ground-consequent relationships.

We can say, ‘Grandfather is ill today because he ate lobster yesterday.’ We can also say, ‘Grandfather must be ill today because he hasn’t got up yet (and we know he is an invariably early riser when he is well).’ In the first sentence because indicates the relation of Cause and Effect: The eating made him ill. In the second, it indicates the relation of what logicians call Ground and Consequent. The old man’s late rising is not the cause of his disorder but the reason why we believe him to be disordered

Rationality depends upon premises being seen as grounds for a consequent conclusion (2.1). But if “causes fully account for a belief, then, since causes work inevitably, the belief would have had to arise whether it had grounds or not” (2.2). If naturalism is true “every event in Nature must be connected with previous events in the Cause and Effect relation” (2.3). Acts of thinking are “events in Nature” (2.4). Therefore, according to naturalism they also must be explained by previous events on a cause-effect basis (2.5). The conclusion (2.6) therefore follows from (2.1) and (2.5).

Premise (3) follows from premise (1) & (2) and logically concludes that the naturalism (philosophical Materialism) atheim is predicated on, should be rejected.