Still, applying symmetry to forces, the third law pinpointed the key feature of forces: all forces are interactions between different bodies. That means that they have the nature of interrelationships of power, efficiency and energy, what could be generalized as a symmetrical force-relationship. In other words, there is no such thing as a unidirectional force or a force that acts on only one body, in one progressive direction. Whenever a body exerts a force F on another body, the other one exerts a force −F on the first body, now in the regressive direction. F and −F are equal in magnitude BUT opposite in direction, having both magnitude and direction as vector quantities.
This action-reaction law, with F called the direct "action", or progressive force, and −F the reverse "reaction", retrogressive or inverse force, is making a decisive inductive case for the most fundamental truth about nature.
The truth of natural causes or forces is that the action and the reaction relationships are universal phenomena applied to all physical interactions and forces, fundamental and non-fundamental.
The Diversity and Unity of the Force-Interaction: the ToE as the Holy Grail of New Physics
All forces are hierarchically related, making up the natural hierarchy of forces, forces, force lines and force fields, topped by the super symmetry force-interaction, a single universal force of coming Theory of Everything (ToE), or Final Theory, Ultimate Theory, or Master Theory, a single, all-encompassing, consistent and complete theoretical framework of physics linking together all the physical properties and phenomena of the universe.
At present, there is no theory of everything that includes the standard model of particle physics and general relativity, such as to calculate the fine structure constant and the mass of the electron, hoping that a deeper search for new particles and dark force at the large particle accelerators could provide critical input for the ToE. There are a number of candidate theories to unite general relativity and quantum field theory, designed as the working theories of quantum gravity: string theory, superstring theory, M-theory, loop quantum gravity, causal fermion systems, and causal sets.
The last two theories are recent developments and in line with our Causal Category Theory of Everything capable to generalize and complete new candidates for a unified physical theory, as causal sets program and the theory of causal fermion systems, considering general relativity and quantum field theory as limiting cases. In the causal sets program, the founding principles are that space-time is fundamentally discrete and that space-time events are related by a partial order, having the physical meaning of the causality relations between space-time events.