It must have been 1974--because it was right around that time that Mark was introduced to the works of C. S. Lewis, and the ideas of this song crystalized in response to his Mere Christianity and Miracles: A Preliminary Study (both of which Mark recommends highly). Within a few years before and after that he wrote a lot of songs on the piano in the key of C (enough that there are several songs on which he forced himself to work in a different key, just to escape the predictability of the piano parts), although this was one of the better progressions overall.
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Being in college, even in a Christian college (the long-gone Luther College of the Bible and Liberal Arts in Teaneck, New Jersey), he encountered much of the philosophy of the age in fuller force than he had previously. Love, it seemed, was strictly a biological motivation to encourage procreation; life had no meaning beyond itself, that is, life existed solely to reproduce life, and no individual was significant in the eternities of the universe. Religion was a collection of outdated notions of ancestors who did not understand their reality. (This is not meant to imply that any of the professors thought or taught such things; students were still aware that this was how the world was perceived by many.)
Lewis had responses to these objections, and raised one more: if indeed the naturalist view of the universe is correct, then our very thought processes are suspect. They are the result of a chain of random events, and irrational causes, whether throwing dice or having the fittest survive, cannot reliably produce rational results. If naturalism is true, we cannot know it, and indeed cannot know anything. The very starting point of any discussion about reality is the agreed assumption that our discussions, our thought processes themselves, are rational. To begin there, either you have to make an unwarrantable assumption and build a house of cards on the back of a moving dune buggy, or you have to accept that there is something beyond nature that provides a basis for our mental processes to be valid.
When he wrote the first verse, he had nothing more in mind by the opening words than that his college education had taught him that love was nothing special--not an eternal connection between people, not the beginning of a meaningful lasting relationship, nothing more than an animal instinct developed to drive procreation and care of offspring and so ensure the survival of the species. He wanted that to have the impact on the listener, that you had to choose between believing that love was the transcendant something that lovers believe and want to believe because there really was something beyond our natural world, versus that relationships were simply animal transactions with no meaning. Finishing the first verse he moved forward into other areas, the search for the meaning of life, the search for God, and similarly put forward the position that without God these were empty notions, that there was no meaning, no morality, nothing beyond the animal. Then when he finished, he recognized that this same principle could apply to thought itself, and carried it to the conclusion that nothing the singer had ever thought, and nothing he had learned, was valid, because thought itself was invalidated by the principle that invalidated love, meaning, morality, and God. This gave an irony to the entire song, as from the opening words it became the case that thought itself was impugned, that "I used to think", "I once thought", but no longer did "I" do so, because thought was meaningless.
Mark often referred to it as "a non-Christian song for Christian purposes", in that it, more than any other he has written, undermines the position of the unbeliever: if you have no belief in some kind of God, nothing that you do believe can be supported. It does not say that directly, nor does it give the alternative; it only attacks the position.
Although it was written during the time of The Last Psalm, Mark only used it in a few solo appearances over the years for a couple decades. It was on the list of songs for Cardiac Output (the lyrics are there), because that was Mark's first attempt at creating a "teaching" band, that is, a Christian band whose ministry was not evangelizing and not leading worship, but encouraging and instructing believers, and this seemed to fit that as a way of showing the strength of our position as against atheism. When 7dB became Collision, he added it because we could do it even if he was the only vocalist, it had something of the driving rock feel (at least, on the choruses) that the band wanted, and as "fifth instrument" Mark could play the keyboard on it. Brittany liked it, and asked to sing it while she was in the band, but it came back to Mark when she left. It thus made the EP, because it was easy enough for everyone to learn their instrumental parts if he sang it.
On the other hand, he felt is was pitched low for him--he could not reliable hit the bottom note at the end, a C, after singing the higher notes, even though it only reaches an E, and particularly in concerts when he was singing yet higher. Besides, thanks to Jeff Zurheide he had been very sensitive about any appearance that he is hogging the solos, so he was looking for ways to sing less and have Jonathan sing more. It would be a tough one for him, trying to sing while duplicating Mark's style of piano playing (he is a much better pianist than Mark, but Mark has a unique and challenging style with his own frills, and Jonathan is working from Mark's scoring of approximately what he plays). Yet once he has recordings of a song Mark tends to see ways to improve the arrangement, and was thinking of adding vocal parts when we finally managed to find Sara, to replace Brittany. The song was the wrong lay for her, so she couldn't take the solo, and it still looked good for Jonathan to have the melody at the end, so Mark rearranged it into three vocals and passed the verse solos around. Thus on the EP it is a solo, but in the 2013 concerts there were three vocals on it.