A recurrent theme in Stoicism (repeated often by Marcus Aurelius) is that nothing is permanent and everything is subject to decay, and so becoming attached to things is a recipe for unhappiness. In one of Seneca’s longer letters (number 65), it emerges that in fact there is something that is permanent, namely Reason (or we could say the Logos).
This point arises from quote a long discussion of the cause(s) of things. Seneca contrasts the Stoic doctrine of causes, in which there is just one cause (Reason), with Aristotle and Plato’s theories. In particular, Plato identified five causes, one of which was the Idea or Pattern of a material thing (I am fairly sure that here Seneca is referring to the platonic Forms).
Seneca describes some interesting features of the Pattern, while arguing (in paragraph 13) that it is not actually the cause itself, but only “an indispensable tool of the cause”. In paragraph 7 he says that patterns/ideas come from the mind of God:
… his mind comprehends the harmonies and the measures of the whole totality of things which are to be carried out; he is filled with these shapes which Plato calls the “ideas,” – imperishable, unchangeable, not subject to decay.
Thus people (men) can die but idea of humanity lives on:
And therefore, though men die, humanity itself, or the idea of man, according to which man is moulded, lasts on, and though men toil and perish, it suffers no change.
As the Stoic God is identical with Reason and we all have a (divine) spark of Reason within us, it surely follows that we ourselves also contain something imperishable and unchanging. Thus while externals are perishable and impermanent, what we have internally, Reason or Virtue, is not. So while we shouldn’t become attached to externals (which are indifferent), we should on the contrary become attached to “internals”, i.e. Reason and Virtue.
I think this is a very therapeutic notion – the injunction to accept change can be quite unsettling, despite counter arguments for why it should not (for instance, that accepting change helps put things into context). I suspect most people have an inner longing for something fixed and secure in their lives. It can’t be things themselves but it can the causes of things, embodying the inexorable laws of Reason.