Any tone can succeed any other tone, any tone can sound simultaneously with any other tone or tones, and any group of tones can be followed by any other group of tones, just as any degree of tension or nuance can occur in any medium under any kind of stress or duration. Successful projection will depend upon the contextual and formal conditions that prevail, and upon the skill and the soul of the composer.
This lovely paragraph immediately establishes the approach used throughout the book. Instead of dictating unsupported rules as though they were chiseled on Sinai, the book catalogues a range of possibilities, weighing each technique’s merits and potentials as a creative resource.
If we extend this approach to all music theory, we end up with a model for practical application. To give an example: the bad way to discuss parallel fifths is to simply say that they are against the rules and leave it at that. The good way is to demonstrate, through musical examples and acoustical principles, how parallel fifths can enrich sonority, but with a cost to textural balance (ie, equality among voices). So, if a student is looking to create a balanced, interweaving texture, s/he should avoid parallel fifths. If, on the other hand, s/he wants thick, sonorous lines, s/he is encouraged to pile on the P5s.
Once you have established that principle, turn to 16th-century counterpoint or 18th-century harmony, explain that we are using those styles to practice creating clean, balanced textures of interweaving parts — then ask whether we should use parallel fifths or avoid them.
Not only will the students get the right answer every time, they will know why it is the right answer — and they will have a reason to care about the distinction.