A second and more important reason for their (St Teresa of Avila and St John of the Cross) preeminence today is that their method of approach to spiritual theology is already very much the modern scientific method as we know it. Both writers take as their starting point an immense quantity of carefully assembled empirical data. They lived at a time and in a place where there was an immense religious zeal, and when innumerable people applied themselves to the business of the spiritual life. Many more, spurred on by fashion rather than piety to study, and in the course of guiding, directing, and praying for them St Teresa and St John of the Cross, observed them and learnt from them, filling out the wealth of their own personal experience from the temptation and the blessings, the successes and the failings of their fellow countrymen and women. Like the true scientists they were, they collated, classified, and examined in detail the information at their disposal, and wrote their conclusions in treaties which deliberately give a conspectus of the whole field of spiritual theology. Thiers is no haphazard, piecemeal work. They aimed to produce a complete and systemized body of doctrine. It was of the genius of St Teresa to pioneer the method, the crowning glory of St John of the Cross to relate the material to the whole background of dogmatic and moral theology of the Church.
…this was no academic exercise. The saints were surrounded by those who wished to make their way to God. The task before them was that of evolving a practical method of guiding them. –E.W. Trueman Dicken ‘The Crucible of Love: A Study of the Mysticism of St Teresa of Jesus and St John of the Cross’