Many things are said to have both the Spirit and the Letter. For example, one says about the Spirit and the Letter of the law, of a political treaty, etc. In principle, anything has both of them. Our Spirit, for instance, is what we want to be. Our Letter is what we are.
Many people assume that there should be no contradiction between the Spirit and the Letter. They reproach anything that contradicts both its Spirit and Letter (say, contradicts the Spirit and the Letter of a treaty). However, in fact, the Spirit and the Letter are contradictory parameters. There cannot be a complete harmony between them. If emphasis is laid on the Spirit then the Letter is fluid and vanishes as such. If emphasis is laid on the Letter then the Spirit vanishes. There are probably states in between.
Take, for example, TRIZ. What is its Spirit? It is a belief that creativity is subject to algorithmization, that there are principles of innovation, which are subject to discovery, etc. What is the Letter of TRIZ ? It is its latest schemes as presented by Altshuller: contradiction matrix, SuField Analysis, etc.
For Altshuller the Spirit was important. That is why he was constantly re-writing the Letter. For some authors of TRIZ-journal, conversely, the Letter is important. They stuck to the numbers: 40 principles, 76 standard solutions, etc. They search for them everywhere. That is why, there is no TRIZ spirit in what they write.