Gurdjieff called his institute the Institute for Harmonious Development. And in the name alone, if we understand properly, we glean clues about his concept of inner work.
Musical theory is part of a complex mathematical system that extends into a series of other areas, such as the periodic table of the elements and the electromagnetic spectrum. These are complex technical areas that take many years to gain a mastery of.
Yet one doesn't need a masters in physics or chemistry, or even music, to know what harmony is. The human being has an innate ability to recognize harmony at once, without any special education. It can be sensed, heard, and understood, without resorting to any fancy theoretical structures or educational processes.
So we know harmony naturally — that is, we can sense it with our being. This is an innate property, that is, something the organisms capable of by itself, before anything else happens. It doesn't need to be taught.
When we refer to harmonious development, we are not referring to development according to the understanding of a complex theory or system. It isn't development according to a special set of secret rules that only some people know. Harmonious development is, rather, learning how to listen; to sense, in an inner way, the harmony of the inner self.
This means that complicated exercises and manipulations aren't actually necessary. Developing sensitivity, developing sensation, is absolutely necessary; but this is a development of the senses in order to render them more perceptive, more attentive to what we are. It isn't about using the body as a bellows to pump it full of prana, which can, in fact, turn out to be damaging. And it isn't about learning a complicated set of rules about stuff.
The idea of harmonious development is related to being able to sense who and what we are: gently, lovingly, intimately, with an attention to ourselves that reminds us we are valuable, and doesn't attempt to whack things with a hammer in order to straighten them out. We can, in a word, knwo ourselves naturally; yet only with a good attention can this happen. That attention is an inner attention, connected to my sensation of myself; and in fact, if I work on this one question, the harmony develops naturally.
This doesn't mean we shouldn't acquire knowledge of why things are the way they are, or attempt to understand them. It simply reminds us that our ability to understand is, in certain areas, inherent. It isn't our ability to understand that is lacking; it's our ability to listen.
And we must use them both.
May your soul be filled with light.