The saints are the ones that know and been able to live the science of holiness.
Yes, we do have modern psychological findings we do use today like lessening our sugar intake to maintain better brain health. But that doesn't mean the science of the saints are now outmoded. The saints know better than we sinners do the perennial truths about how to be holy. Caritas in Veritate, art. 76, where Pope Benedict XVI reminds us that
One aspect of the contemporary technological mindset is the tendency to consider the problems and emotions of the interior life from a purely psychological point of view, even to the point of neurological reductionism. In this way man's interiority is emptied of its meaning and gradually our awareness of the human soul's ontological depths, as probed by the saints, is lost. The question of development is closely bound up with our understanding of the human soul, insofar as we often reduce the self to the psyche and confuse the soul's health with emotional well-being. These over-simplifications stem from a profound failure to understand the spiritual life, and they obscure the fact that the development of individuals and peoples depends partly on the resolution of problems of a spiritual nature. Development must include not just material growth but also spiritual growth, since the human person is a “unity of body and soul”[156], born of God's creative love and destined for eternal life...
I've tackled spiritual readings before and there are lots of good sources of classical spiritual readings nowadays online one can download for free, aside from buying new writings.
Sin have been won against by these saint writers. They surely know something we sinners don't. Such a science of praxis gives us certainty against mere theories, knowledge, even unproven facts, and wrong scientific findings of scientists not specializing on the field of chaste living.
