τὸ μακάριον καὶ ἄφθαρτον οὔτε αὐτὸ πράγματα ἔχει οὔτε ἄλλῳ παρέχει· ὥστε οὔτε ὀργαῖς οὔτε χάρισι συνέχεται· ἐν ἀσθενεῖ γὰρ πᾶν τὸ τοιοῦτον.
"The blessed and imperishable being [τὸ μακάριον καὶ ἄφθαρτον] has no troubles itself nor causes troubles for others; as a consequence, it is affected by neither anger nor gratitude; because all this would be an indication of weakness, sickness, or lack of strength." (Translation is my own)
Being affected by anger as a sickness or weakness makes sense, but why would being affected by gratitude be a sign of weakness? One conjecture would be that it would show a lack of self-reliance / αυτάρκεια. If we needed reassurance / affirmation from others and didn't just do things because they were pleasurable, we're not truly living a blessed life. That sense of self-assurance would make one a μακάριος καὶ ἄφθαρτος. Blessed, yes. Imperishable? This echoes an idea that, once unnecessary desires are uprooted, they can't come back. But that's a Buddhist concept. Could Epicurus have really been implying that kind of Imperishability?
Some of the senses of φθαρτος, the opposite of ἄφθαρτος, are "pass away, able to be bribed, adrift." Considering the opposite of these qualities - "not pass away, not able to be bribed, not adrift" - gives a deeper sense to what a mortal life potentially filled with these senses of ἄφθαρτος would be like. We know we are mortal. Epicurus and Lucretius proclaim that. So interpreting how we mortals can be imperishable beings isn't readily apparent at first. But this doctrine - all the Principal Doctrines - have to be applicable to our lives, otherwise of what use are they to us? Epicurus was adamant that philosophy had to be practical and to improve one's life. Re-examining the connotations of ἄφθαρτος allows us to see that that idea doesn't have to be a mystical iimperishability but one rooted in the here and now.