John Sellars in The Art of Living (pg. 140):
"According to Stoic ethical theory, of the impulses towards action, the primary impulse (πρωτη ορμη) is towards self-preservation. This leads one to select things that are in accordance with one's own nature (κατα φυσιν), such as food or anything else conducive to one's health. Any action that is in accordance with one's nature (κατα φυσιν) may be said to be an 'appropriate action' (καθηκον). Many actions inspired by this primary impulse are common to animals, infants, and adults. However, for a rational adult the only properly appropriate actions will be those which are the product of rational impulses, namely an impulse with a rational justification. Thus they will be actions that are appropriate to one's nature not merely as a biological entity but also as a rational being. Some of these appropriate actions will be unconditional; others will vary according to circumstance."
From Diogenes Laertius: Lives of the Eminent Philosophers, translated by R.D. Hicks:
"An animal's first impulse, say the Stoics, is to
self-preservation, because nature from the outset endears it to itself, as
Chrysippus affirms in the first book of his work On Ends: his
words are, "The dearest thing to every animal is its own constitution
and its consciousness thereof"; for it was not likely that nature
should estrange the living thing from itself or that she should
leave the creature she has made without either estrangement from or
affection for its own constitution. We are forced then to conclude
that nature in constituting the animal made it near and dear to itself;
for so it comes to repel all that is injurious and give free access to all
that is serviceable or akin to it.
"As for the assertion made by some people that pleasure is the object to
which the first impulse of animals is directed, it is shown by the Stoics
to be false. For pleasure, if it is really felt, they
declare to be a by-product, which never comes until nature by itself has sought and found the means suitable to the animal's
existence or constitution; it is an aftermath comparable to the condition
of animals thriving and plants in full bloom. And nature, they say,
made no difference originally between plants and animals, for she
regulates the life of plants too, in their case without impulse and
sensation, just as also certain processes go on of a vegetative kind in
us. But when in the case of animals impulse has been superadded,
whereby they are enabled to go in quest of their proper aliment, for them,
say the Stoics, Nature's rule is to follow the direction of impulse.
But when reason by way of a more perfect leadership has been bestowed on
the beings we call rational, for them life according to reason rightly
becomes the natural life. For reason supervenes to shape impulse
scientifically" (7.85-6, trans. Hicks).
Long and Sedley comment:
"The 'appropriate' object of an animal's first impulse can thus be described as 'self-preservation'. In [para. 1] this is expanded into 'its own constitution and the consciousness of this'...The Stoics may have defined 'appropriation' as 'perception of what is appropriate'...Why this should be thought to require self-consciousness, however rudimentary.., may seem inadequately explained in their reasoning. But not too much weight need or should be attached to the term 'consciousness'. It is an attempt, and an interesting one, to do justice to data which would now be explained by reference to natural selection and genetic coding" (pg. 351).
Diogenes Laertius. Lives of eminent philosophers, with an English translation by R. D. Hicks. (1925) Vol. 2. Cambridge MA: Harvard University Press.
Long, A. A., Sedley, D. N. (1987). The Hellenistic
Philosophers: vol. 1. translations of the principle
sources with philosophical commentary. Cambridge,
England: Cambridge University Press.
Sellars, John (2003). The Art of Living: The Stoics on the Nature and Function of Philosophy. Aldershot: Ashgate.