A source is always a whole, a non-manifesting couple of interacting entities, one active/creative, a force, and one passive/receptive, a field, potentially or effectively producing action, events. So, a source is, at the same time, the causal origin (what acts) and the variational substrate (what varies) of any event. Force and field are a unity, different aspects, but unitarian and indivisible expressions of a source. The Force is the active, the creative side of the Source. The Field is the passive, its receptive side. The Source express itself in these two entities, the unmanifested, root of any manifestation, physical and cognitive, the everything, the whole Reality, the Universe in evolution. I intend the unmanifested as the absolute, the pure and formless ground of being from which creation and the manifestation arise. The Source manifest itself in the complementary expression (that means “pressure to existence”) of a unitarian couple of interacting entities, Force and Field. The Source in itself, as the One, is free from change, a concept close to the “unmoved mover”, or “uncaused cause”, but its direct manifestation, The Action, is the change in itself. The absolute/unmanifested and the relative/manifest are different levels of the same reality.

A force is an agent, a cause, an entity that, interacting with a field, produce existence, the causal side of a source of something that exists. For causal side, I intend what relates to the expression of a force acting on a field producing events, action. It is an internal interaction between the two unitary aspects of a source. A unity, a self-manifestation of a source through this internal interaction.

A field is an acted, the former “substance”, the substrate, the variational side of a source. For variational side, I intend the aspects of a source that can vary in space (relative space location  position and its derivatives with respect to space and/or in time) and/or in time (relative time location  instant and its derivatives with respect to space and/or in time). The basic variational aspect of Reality, that correspond to the primary mode (Perturbation) of expression of the Action, the dynamics of the Elementary Events, is space-time itself, the metric field, seen as the dimensional counterpart of mass density and the gravitational phenomena. The others variational aspects, that correspond to the further, more complex, modes of expression of the Action, are attraction/repulsion related to motion (Translation), vorticity/chirality related to charge (Rotation) and orientation of the Rotation axis (Rotation Axis) related to the electromagnetic phenomena, all the most complex nuclear phenomena, forces and fields related to the combination of these fundamental modes and the above mentioned physical quantities and phenomena.

These two sides of the source, force and field, interacting, manifest their specific features and at same time acquire the features of the other different side.
The field manifests its passive feature of “resistance” to variation, reacting to the pressure of the force. In turn, this reaction manifests itself as a force opposed to the original force. It opposes as a counter-pressure to the action of the force.
The force manifests its active feature of “pressure” for variation, reacting to the resistance of the field. In turn, this reaction manifest itself as a variation of the intensity of the force. It opposes as a counter-resistance to the reaction of the field.
So, the force has in itself a secondary/derived aspect of variation/resistance and the field has in itself a secondary/derived aspect of force/pressure.
The best image I can find to represent this fundamental interconnection between the two opposites in the basic nature of a source is the ancient glyph of Yin Yang. I know, this can be seen by a scientist as an indebted reference to a world of beliefs and superstitions. I don’t think so. Anyone who wants to walk the paths of knowledge must have the humility to observe with respect every expression of the human thought, aware that even the most insignificant concept, produced by a man, can contain a fund of truth and can be useful to reveal aspects of the Reality. In this case, then, we must bow our heads in front of one of the highest peaks of human knowledge, the foundation of some of the most important philosophies of the East.

Pressure and resistance together embody the unity in the expression of a source, because pressure implies a force, the causal side, and resistance a field, the variational side.

In the classic concept of pressure, we find a force and an area of space. In the hypothesis of a rigid body containing a volume of something, what varies is the extension of the volume of space. This in the classic physical world. At the most elementary level of Reality, in the very fabric of space time and the fundamental physical phenomena, what varies is the compression/dilatation of the space-time itself (space distances, areas, volumes and time distances/periods), effect of the major or minor density of the fundamental variations of the Field produced by the Force.

Conversely, in the classic concept of resistance, we find a substance, a material, a physical body, a continuous body, in which a deformation results from a stress induced by applied forces. Resistance is itself a force, a force that tends to oppose to variation, that limits the action of a force. This in the classic physical world. At the most elementary level of Reality, where the action, the variation, is relative to the compression/dilatation of the space-time, we see the resistance as the capacity of the field to oppose to this fundamental deformation of the space-time itself. Without resistance, no reaction. A finite force will cause an infinite action. Conservation finds its limit, in which the action lost itself in an endless variation in space and time. Space and time couldn’t exist and so causality.