Short answer: Nature is governed by the logos which is based on reason.
Long answer:
- Nature.
Nature comprises all things that are in existence. The world or universe or simply everything. More importantly, nature is all materials, that is corporeal things, dead and living as well as divine things. There is no distinction between the ‘non-divine’ and the ‘divine’. Nature is all materials, nature is all gods or the god, whatever you prefer.
Applying the stoic virtues is the foundation of living a life according to nature. That is to be a functioning human being that lives in a state of eudaimonia (εὐδαιμονία). This describes someone who focuses on the development of his own virtues. Someone with fully developed virtues is the very definition of someone that is beneficial for mankind. So, living in accordance with nature means to develop someone’s virtues which is the same as being beneficial for mankind.
- Logos.
In general, the Logos (λόγος) describes the underlying principle of the world or nature or kosmos (κόσμος). This order is governed by reason. Thus, the logos is seen as the active form of reason that pervades and governs the universe. Logos is the universal reason.
It can be regarded as an all-encompassing divine principle, in other words the work of the gods, of the creator, physics etc.
This idea goes back to Heraclitus. Although Heraclitus was not a stoic, in fact he died a century before Zeno of Citium was born, he coined the term Logos. For him, the Logos was a cosmic law based on reason.
Change is a constant, change is essential for the universe. Nothing persists forever, since everything is governed by change that means: All things are prone to collapse and destruction to be reborn and renewed again. Things die, so that other things can be born.
This does not mean, however, that things can be created out of nothing, that there essentially is nothing. Change does not mean things appear out of nothingness (as Parmenides insisted). Change means that all things are the result of and will result in conversion. This implies that all elements are part of an ongoing process of evolution, revolution and metamorphosis.
We live in a world of change. Since all things and beings are prone to change, all things and beings are united by change. United by the logos.
Heraclitus described the world in terms of a river:
“No man ever steps in the same river twice, for it’s not the same river and he’s not the same man.”
-Heraclitus; DK 22 B 12
From this stems the stoic leitmotiv of time being described as a river:
“Time is a sort of river of passing events, and strong is its current; no sooner is a thing brought to sight than it is swept by and another takes its place, and this too will be swept away.”
-Marcus Aurelius
The Logos can also be found at other parts of the Meditations:
"Look at everything that exists, and observe that it is already in dissolution and in change, and as it were putrefaction or dispersion, or that everything is so constituted by nature as to die. "
-Marcus Aurelius; Meditations; X, 20
Or:
“Is any man afraid of change? Why what can take place without change? What then is more pleasing or more suitable to the universal nature? And canst thou take a bath unless the wood undergoes a change? And canst thou be nourished, unless the food undergoes a change? And can anything else that is useful be accomplished without change? Dost thou not see then that for thyself also to change is just the same, and equally necessary for the universal nature?”
-Marcus Aurelius; Meditations; XII, 18
Or:
"Loss is nothing else than change. But the universal nature delights in change, and in obedience to her all things are now done well, and from eternity have been done in like form, and will be such to time without end. What, then, dost thou say? That all things have been and all things always will be bad, and that no power has ever been found in so many gods to rectify these things, but the world has been condemned to be found in never ceasing evil? "
-Marcus Aurelius; Meditations; VIII, 35
- Reason.
Reason describes the rationality of the world and on the other hand the ability of an individual to think logically. The goal of a stoic thinker is to understand the universal principle that acts according to reason (Logos).
The major takeaway here is that everything that happens in nature can ultimately be understood and made sense of by rational, logical thinking. In other words: reason.
Nature is therefore based on cause and effect relations: Every cause has an effect and every effect has a cause. No effects can happen without causes.
This means that nature is deterministic. Nature can be understood in terms of logical thinking, by applying reason. Nature is governed by reason.
Virtue is the sole good. And since virtue is applied wisdom which is based on reason, stoic ethics comes down to applying reason.
TL;DR: If the universe (or nature) were a computer, the Logos would be its operating system, with reason being its programming language. In order to understand the universe, you have to understand that its operating system is based on reason. If you accept that nature is governed by the logos, you have to accept that everything changes, change is a constant of the universe. If you understand reason, you accept that the universe is deterministic and based on cause and effect relationships.
Source: https://www.reddit.com/r/Stoicism/comments/6akp9s/what_are_logos_nature_and_reason_in_stoicism/