The point that Lennox is trying to make can be illustrated in the following manner: Suppose thousands of years ago a group of primitive individuals came across a leather bound book filled with letters and symbols the likes of which they had never seen before. If the individuals were reductionist they would only try and explain the nature of the book in terms of the ink and paper.
Though the specific semantics of the symbols might not be deducible, we certainly do as rational and logical creatures have the capacity to recognize the product of other minds. Of course the symbols are abstract but we still have the capacity to infer intelligent causation. This can also be understood through logical syntax, where we recognize the ordered structure of things as meaningful, again this is part of being a rational and logical human being.
In fact the scriptures tell us that all of creation – the sun, moon and stars – are symbols with a semiotic dimension, they are a universal language that denote the existence of a creator.
Psalms 19: 1-4
“The heavens declare the glory of God; And the firmament shows His handiwork.
Day unto day utters speech, And night unto night reveals knowledge.
There is no speech nor language Where their voice is not heard.
Their line has gone out through all the earth, And their words to the end of the world.”
As Christians we hold that there is a semiotic dimension to the world around us where we can reasonably infer intelligent design, in the same way that primitive individuals finding a leather bound book with mysterious and foreign symbols inside of it would.