Any human activity needs a 'measure' to validate appropriate development. Soul creation is no exception. It requires tangible, verifiable evidence that the seeker can use as a barometer of progress.
Only when the physical body (the lower Soul) has sufficiently 'matured' (normally around the age of 18-25 years) can Soul creation become a 'potentiality'.
The word 'maturing' or 'mature' is, in many ways, used as a substitute for the idea and process of 'Soul Creation' in modern language.
The first sign that one is progressing to support harmonized Soul creation is when one has consciously developed the ability to intentionally 'relax' any part of the body unaided. This begins with relaxing parts of the body at first, then progressing to sustain a whole relaxation of the body 'at will'.
Very few people are up to the task of becoming a genuine seeker of Soul creation, and most genuine seekers never get as far as the first sign - indicating just how difficult it is.
Conscious and total relaxation of the body takes years of practice under all sorts of conditions. Once mastered, the natural energies of the body become liberated to be used for Soul formation rather than 'leaking away' into unnecessary tensions and anxieties.
The development of the conscious ability to relax the body is never done in isolation of the other signs and abilities required for Soul Creation. However, a relaxed body is the first 'concrete' and 'visible' sign along the way that cannot be artificially fabricated.