Concerning Cassius ' feedback:
SOE15: Under normal circumstances, we are in control of our mental dispositions.
Objection to SOE15: The "under normal circumstances" probably is so ambiguous that it negates any benefit from this tenet. The Epicurean point in my understanding is that we should work to remain in control of our mental dispositions (like we work to control everything else) so that we maximize pleasure and minimize pain. By mentioning mental dispositions without really stating anything significant about them, the implication is that you are endorsing some kind of Stoic mind control that leads to suppression of emotions. Presumably you would only want to suggest that painful emotions should be kept under control, but even that would likely be a non-Epicurean interpretation, since it is recorded in DIogenes Laertius that Epicurus said that the wise man feels his emotions more deeply than others, and this is no hindrance to his wisdom.
I was mainly thinking of Fragment 112 Diogenes, which states that the “sum of happiness is our disposition, of which we are masters". I considered this against PD 20 in one essay, so this is not a Stoic insight at all. The goal of each Tenet is to start a more in depth conversation and commentary on each, not to close the discussion where the Tenet ends (as I mention in the introduction of the Tenets, where I discuss the problem of over-simplification).
When Philodemus addresses habitual fury or arrogance as moral diseases, he also refers to it as diathesis (a bad disposition) which needs to be replaced by a better, friendly, kind, disposition.
So diathesis / dispositions are an important concept in moral development, and they deserve further discussion.
I say "under normal circumstances" because of problems like drug use, addiction, and some mental health issues that I am not fully an expert on, but last night my neighbor texted me because he had a panic attack, and sometimes people (like when they lack sleep) can lose control of their dispositions. There are people who feel, maybe at times, unable to control their disposition. I can also think of diabetes and the emotional / mental problems that diabetes can generate. So it seems to me that, if we do not take care of our health (which is a natural and necessary pleasure), this affects our habitual disposition.