To explain what I mean by "modern medical science has no underlying theoretical basis," let me offer an example of a non-modern medical system that has such basis. Not to prove that it's a "right" or "wrong" theory, just to give anyone interested an idea of what a "theoretical basis of medical science" CAN be like when it's there, as opposed to what it's like when it is nonexistent. The theoretical basis of traditional Chinese medicine is the concept of energy transformations. It postulates that we live in a world of manifest phenomena that share a common origin, and that everything that happens in this world.
A) Happens in rhythmical cycles - be it planetary rotations around the sun or seasonal changes on this planet or blood circulation cycles in the human body or circadian and monthly hormonal changes, etc.
B) These cyclic changes work due to waxing and waning of the basic types of energies of the world. For convenience, the two opposite and complementary types of energies interacting in the universe are given verbal labels that point towards the nature and quality of a particular phenomenon under scrutiny. The waxing-increase-speed-up-opening type phenomena are dubbed "yang" and the waning-decrease-closing-slow-down type, "yin"
C) Increasing yin-type manifestations of any phenomenon or entity decreases its yang-type manifestations, and vice versa
D) Energies of the world, whether "material" or "immaterial" (i.e. "pattern- or tendency-forming," function-related) manifest as "vitality, energy, spirit" (Chinese jing, qi, shen) and undergo transformations, vitality to energy to spirit, or the other way around, in certain orderly patterns which, when disrupted, cause stagnation, blockage or "entanglement" of these energies
E) The basis for smooth operations of the transformations in the body is memory, which is part of jing and consists of the body's inherent idea of what it is and how it must go about being what it is (e.g., a tree knows how to be a tree and not a mouse due to jing, the nature of its original vitality; this knowledge includes expertise in dynamic homeostasis - how to be a functioning, living organism of a particular kind, and what to do and what not to do on every level at every step of any and all transformations that the whole entity will undergo during its lifetime)
F) There are five types of energy involved in all transformations; they are known as the Five Phases, and operate cyclically, via one nourishing the next, the next decreasing the former, and each controlling the one derived from the one it nourishes. The cycles go on smoothly in a healthy body, get disrupted in an unhealthy body. Because the process is cyclic, disrupting one "phase" of energy will affect the next and this one, the next, and ultimately all
G) Transformations of energies are directional in space and time, and are reversible in space and time
H) The factors influencing the nature and quality of transformations are known as "resonance" (Chinese gan ying) - meaning phenomena of the outside world "resonate" with those inside the human being and prompt transformations of a certain kind going in a certain direction. Example: the setting sun and advancing darkness will cause a shift in the yin-yang balance (hormonal, metabolic, sympathetic-parasympathetic, etc.) from daytime "more yang" to nighttime "more yin" dynamic equilibrium in the human body, following ("in synch with, resonating with") similar developments in the ambient medium
I) There's no absolutes, nothing is "pure this" or "pure that," phenomena of the manifest world and the human body alike are meaningful only in comparisons; thus, nothing is "yin" and nothing is "yang," one can talk only of "more yin" vs. "more yang." Example: a healthy male has a preponderance of male "yang" hormones (testosterone) but also, invariably, some "yin" female hormones (estrogen) at every step of his live functioning; and vice versa
J) A trained medical practitioner learns multiple methods of discerning disruptions of normal smooth cyclic operations in the body. These, when present, affect all organs and systems, some more explicitly and others, in a more "hidden" fashion; however, there's clusters of manifestations where they can be discerned quite clearly by an appropriately trained professional and so on.
This is the tip of the iceberg, of course, but from the above you can see that the whole system is tightly consistent with its underlying basic principles, and doesn't leave any phenomena outside its overall power to discern, understand, classify, address, "know the reasons why" and "know what can be done about." Which is more than can be said about the Western scientific models, of which there are scores, all of them at odds and/or in conflict with scores of other Western scientific models.
Which is one reason I read two-thousand-year-old medical books in order to "get it" on the conceptual level, with subjectively better overall cognitive results than I derive from any "cutting edge research," whether "alternative" or "orthodox." Maybe I'm just not smart enough to "get it" when it is as fragmented as Western science is. Or maybe I'm just an integration junkie...