From a letter to Clyde S. Kilby, May 7, 1959, from C. S. Lewis

Whatever view we hold of the divine authority of Scripture must make room for the following facts:

1. The distinction which St. Paul makes in 1 Cor 7:10-12 between /ego all' ho kurios [not myself but the Lord] (v. 10) and [I myself say, not the Lord]. (v. 12)

2. The apparent inconsistencies between the genealogies in Matt. 1 and Luke 3; with the accounts of the death of Judas in Matt. 27:5 and Acts 1:18-19.
{see Genealogies}

3. St. Luke’s own account of how he obtained his matter [Luke 1:1-4]. {¹ Many have undertaken to draw up an account of the things that have been fulfilled among us, ²just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. ³Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, ⁴so that you may know the certainty of the things you have been taught.}

4. The universally admitted unhistoricity (I do not say, of course, falsity) of at least some of the narratives in Scripture (the parables), which may well also extend to Jonah and Job.

5. If every good and perfect gift comes from the Father of lights [James 1:17], then all true and edifying writings, whether in Scripture or not, must be in some sense inspired.

6. John 11:47-52 Inspiration may operate in a wicked man without him knowing it, and he can then utter the untruth he intends (propriety of making an innocent man a political scapegoat) as well as the truth he does not intend (the divine sacrifice).

It seems to me that 2 and 4 rule out the view that every statement in Scripture must be historical truth. And 1, 3, 5, and 6 rule out the view that inspiration is a single thing in the sense that, if present at all, it is always present in the same mode and the same degree. Therefore, I think, rule out the view that any one passage taken in isolation can be assumed to be inerrant in exactly the same sense as any other: e.g., that the numbers of O.T. armies (which in view of the size of the country, if true, involve continuous miracle) are statistically correct because the story of the Resurrection is historically correct. That the over-all operation of Scripture is to convey God’s Word to the reader (he also needs his inspiration) who reads it in the right spirit, I fully believe. That it also gives true answers to all the questions (often religiously irrelevant) which he might ask, I don’t. The very kind of truth we are often demanding was, in my opinion, not even envisaged by the ancients.

{} Added here for explanation.

Quoted in Michael J. Christensen, "C. S. Lewis on Scripture", Abingdon, 1979, Appendix A.

C. S. Lewis on Inerrancy, Inspiration, and Historicity of Scripture