Chap.1
"What we learn from experience depends on the kind of philosophy we bring to experience" C.S.L.
How do we know anything? How do we learn anything? We must follow the laws of thought, the principles essential to knowledge: law of noncontradiction, law of causality, assent to the basic reliability of sense perception, and the basic reliability of the use of language to communicate our ideas. These principles are the foundation of how the human mind obtains knowledge--they flow out of human reason.
He seems to be saying that our explaination of reality is full of philosophical implications. He mentions a friend who saw a ghost, but does not believe in the immortal soul. So she explained the experience in terms that she did believe: "it was an illusion, a trick of the nerves." In other words, we bring our conclusions, and unscientifically reason backwards, manipulating the evidence to prove our conclusion true. What does Lewis actually want us to put aside? Is it possible to leave our philosophies outside, so to speak? Maybe he is telling us to examine our philosophies first, then, your understanding of experience will be closer to the truth? But how will we know that our philosophies are true? They must follow the principles of thought.