Some philosophers say everything is process. From the cosmic scale of the universe to the submicroscopic scale of atoms, physical forces marshal matters and energies to and fro. In between these at the human scale, biology is ruled by flows of energy from the sun and nutrients from the environment as well as from other living beings. Dynamical forces such as temperature, weather and tides also affect biology and even the cultural processes of higher lifeforms.
At the scale of the solar system, gravity collects gases to ignite stars and form planets. Once stars reach their limits to burn, gravity can collapse them to dense cinders and black holes or even to rebound and spread their atomic matters in novas and supernovas. Even light spread by the stars can be gathered together into gravity wells such as black holes. Stars in turn are gathered into galaxies by the gravity of black holes and even unknown dark matters.
At the scale of atoms and molecules, temperature differentials and water can partition certain types of elementary constituents to form membranes and segregate insides from outsides. If an inside is protected sufficiently, then there is time and the conditions to harbor and perpetuate the delicate structures and processes that form cells. Cells can even gather together and continue as multicellular communities, or only temporarily to fruit and disperse again as simple creatures known as slime molds.
At the individual human or societal scale, there are flows for nutrients and excreta, materials for habitation and the manufacture of tools, distributed energies such as electricity, fossil fuels, and information for learning, work, and civic participation. Even speech and writing can be thought as flows of information. But just as flows of nutrients and materials and energies can prove toxic to biological health or ecologies, so can information.
For two-dimensional dynamical systems, certain common elements can be mapped out: sources and sinks, saddles and centers. Sources have flows out from a point or region, and sinks have flows in. Saddles have a roughly stationary center, due to balanced flows in and out at (not necessarily) right angles. Centers are circular vortexes about a stationary point or region. Sources and sinks can spiral, saddles can twist, centers can become eccentric or elliptical.
For example, think about everyday weather forecasts. The atmosphere is relatively thin compared to the earth and so the flows of air can be considered two-dimensional, at least at the ordinary strata of human habitation. There are air pressure highs and lows (sources and sinks), and air temperature cold and warm fronts (usually not saddles though), stationary fronts (centers?), and even circulations (hurricanes are spiraling sinks I guess).
Ordinary, human-sized change has conditioned many of our intuitions and insights about the way the universe works. Heraclitus famously said that all was change, and so he thought fire was the primal element. His predecessor Thales thought that water was instead the basic element, and it is pretty mutable also. Lucretius, inspired by Empedocles, thought none of the four classical elements were foundational, but all were composed of tiny bits that fell and bounced against each other through an endless void.
As earth is in opposition to air, and fire to water, the seasonal changes of temperature and moisture were considered by Hippocrates. Heat gains dominion over cold in Spring and Summer, but cold replaces it in Fall and Winter. Similarly wet and dry quarters cycle through the seasons. These oppositions gave rise to the theory of the four temperaments or humourism. Even to this day these considerations have inspired various theories of personality, like the Myers-Briggs Assessment.
Is everything a struggle of opposites? Empedocles, already mentioned, thought love and strife were the relations that respectively attracted and repelled all matter in their dance and change. Now we know that things fall towards the earth, not for the love of it, but because of the shape of space that the earth’s mass makes. Heat flows into the cold because both even up. Order dissolves into chaos since the latter is more likely, unless fed by other sources of order turning to disorder.
Is everything a flow between opposites? Light spreads out and diminishes into darkness, but gravity gathers matter together. Enough gravity can even gather light and bind it into the darkness of a black hole. A drop of ink spreads out in a glass of water, never to return to that inky state, unless the glass sits and the water evaporates until only a drop remains. Even epidemics and pandemics can be thought to be flows of transmission and contagion. Here, the small becomes the large, and the few the many.
Also note that these four classifications are somewhat analogous to four valued logic: True is Source, False is Sink, None is Center, Both is Saddle.