In any technical field, there are both a general statement of goals and principles, as well as specific steps that should be applied to achieve those results. In many endeavours, particularly those involved in dealing with the physical world and the science that operates in that world, the process is relatively straightforward and consistent and anyone applying those steps would expect to achieve the stated end result. Thus, we rely on airplanes, using principles enunciated by mechanical physics and aerodynamics, to be able to take off and fly around the world. The methodology is set and as long as the steps are fulfilled, the airplane does what it is expected to do.
There are any number of cookbooks that provide specifics as to ingredients, quantities, manner of preparation, time of cooking, etc. to achieve the desired meal, baked goods or dessert. Anyone following the precise instructions can be expected to wind up with the end result described, although even here, some variation does in fact occur due to specific individual or local factors.
Similarly, although not identically, the yogic sciences have been reduced to specific steps and principles for achievement of stated objectives. It is anticipated that just about anyone following the steps outlined can get to the result. The difference here is that each individual is starting from a different place in their evolutionary cycle, each one has a different set of factors in their own lives and in their environment, and each one is subjected to unique pressures. This makes it difficult, if not impossible, to lay down specific steps for everyone to follow. Yoga is not quite the same as following a recipe and baking a cake! What can be done is that the basic objectives and principles of action can be set forth, and specific options outlined so that the seeker can apply them at the right time, in the right circumstances and in the right sequence to meet their specific needs. Rules therefore are insufficient without an understanding of the need for flexibility and a certain degree of individual interpretation, as long as the basic principles are kept before one’s eyes.
Sri Aurobindo notes: “Another kind of Shastra is not Scripture, but a statement of the science and methods, the effective principles and way of working of the path of Yoga which the Sadhaka elects to follow. Each path has its Shastra, either written or traditional, passing from mouth to mouth through a long line of Teachers. In India a great authority, a high reverence even is ordinarily attached to the written or traditional teaching. All the lines of the Yoga are supposed to be fixed and the Teacher who has received the Shastra by tradition and realised it in practice guides the disciple along the immemorial tracks. One often even hears the objection urged against a new practice, a new Yogic teaching, the adoption of a new formula, ‘It is not according to the Shastra.’ But neither in fact nor in the actual practice of the Yogins is there really any such entire rigidity of an iron door shut against new truth, fresh revelation, widened experience. The written or traditional teaching expresses the knowledge and experiences of many centuries systematised, organised, made attainable to the beginner. Its importance and utility are therefore immense. But a great freedom of variation and development is always practicable. Even so highly scientific a system as Rajayoga can be practised on other lines than the organised method of Patanjali. Each of the three paths, trimarga *(the triple path of Knowledge, Devotion and Works.), breaks into many bypaths which meet again at the goal. The general knowledge on which the Yoga depends is fixed, but the order, the succession, the devices, the forms must be allowed to vary; for the needs and particular impulsions of the individual nature have to be satisfied even while the general truths remain firm and constant.”
Sri Aurobindo and the Mother, Growing Within: The Psychology of Inner Development, Chapter III Growth of Consciousness Basic Requisites, pp. 48-49