The verse "And you will be acting, I think, still more senselessly, if you follow vain conceits about the truth" from 4 Maccabees 5:9 is essentially warning against being led astray by arrogant or empty ideas about the truth, rather than focusing on real understanding and righteous action.

Here's a breakdown of its meaning:

"And you will be acting, I think, still more senselessly...": This indicates the speaker (likely a character within the narrative) believes that the actions or intended actions of the person being addressed are already unwise, but that further deviation would be even more foolish.
"...if you follow vain conceits...": This refers to empty, puffed-up, or egotistical ideas. "Vain" suggests these ideas are without real substance or merit, driven by ego or pride. "Conceits" are elaborate or excessive notions, often implying a lack of groundedness in reality.
"...about the truth.": This specifies that these vain conceits are concerning the understanding or pursuit of truth. The truth in this context is likely related to moral and religious principles central to the narrative.

In essence, the verse suggests that following one's own inflated or shallow ideas about truth, instead of adhering to wisdom and/or divinely revealed principles, will lead to even greater foolishness and wrong actions. It's a caution against intellectual arrogance and a plea for humility in the pursuit of genuine understanding. The character is essentially saying, "You're already on a bad path, but you'll make things even worse if you let your ego and your own empty ideas about what's right guide you."

In the context of 4 Maccabees, this probably relates to the virtue of reasoned piety and the rejection of temptations that lead to the abandonment of religious law. Following "vain conceits" could represent yielding to temptations and rationalizing those choices with superficial or self-serving arguments, rather than adhering to the steadfast principles of faith and reason.