One such topic is that of secular morality. Whenever we attempt to explain how secular morality works, we're then treated to a slew of "oh yeah, but what about...?", where they attempt to debunk the concept of morality by trying to establish that it, too, suffers from an infinite regress of justifications.
I have a opinion about that.
The conversation usually goes something like this:
"How can you have morality without GOD?"
I explain that secular morality is an objective analysis of whether particular actions are increasing benefit, and reducing harm.
"But how can you know what's harmful or beneficial?"
Unphased, I detail how if we're talking about whether falling into a vat of acid is harmful or beneficial, if the biological unit stops functioning, that's considered bad.
"But why do you care whether harm is bad or not?"
This is the point of contention. They keep trying to poke holes in what I'm saying, until there's an opening, and they can throw their arms into the air, screaming "CHECKMATE!"
If you aren't familiar with the Is/Ought problem, it's relatively straight forward. Just because something "is", doesn't mean that it implies that we "ought" to do something (or "ought" not to do so).
Atheists are pummeled with one particular example - if evolution is true, then we ought to engage in eugenics (or so-called "Social Darwinism"). It's a ludicrous notion. Another example is the idea that because rape is natural, that it's therefore a justification for why it's "not so bad" for humans to do it.
If you haven't identified it yet, this frequently manifests as a "Naturalistic Fallacy."
Back to the question of secular morality, up until that point in the conversation, I was discussing the philosophical justifications as to why secular morality works in a particular way. Philosophy isn't mental masturbation. It's a literal application of critical thinking - figuring out how to approach common problems/issues in life. In this case, we're trying to figure out how to successfully operate this "secular morality."
At the point where I'm asked "why do you care?", we've actually left philosophy behind. The "ought" is tearfully waving back to us on the docks, as our boat sails away... while we venture out into the great world of "is." No longer is secular morality justified by philosophy, but rather by happenstance.
Thanks to this natural mechanism, we have a number of instincts built into us, such as the "survival instinct", and more specifically to social species, "empathy", which is the glue that holds a society together. We couldn't operate without it.
I don't favor benefit over harm because I've philosophically justified it. I favor it because my brain was wired that way, incidentally by what aided our lineage to survive (pleasurable things are usually beneficial - like high calorie foods, and painful things are typically harmful - like getting burned).
It's simply what is. Our preference for benefit over harm needs no more philosophical justification than a rock would need for being at the bottom of a hill, rather than the top. The mechanisms of the universe simply produced a configuration of the universe, that is.
I can hear the objection already - the notion that this renders the morality arbitrary, and meaningless. This seems like a misreading of the implications.
If reducing suffering, and promoting happiness isn't relevant... what is? If you don't agree with me on that, I don't even recognize what you think morality is, anymore. I think you are the one who has a sense of morality that's devoid of meaning and relevancy.
Regardless, that's where secular morality starts - pain, pleasure and biology.
AddendumI realized later that I failed to connect my point with the "problem of an infinite regress."
Philosophically, the regress ends, clearly... but the chain of events going back may still have that problem, when considering biology, chemistry, physics, etc.
No, I say! The reason for this is because of how we approach science as a whole, and justify our knowledge.
Science doesn't start with the premise that "It's turtles all the way down", and then builds extra layers on top of that. Science begins with the observation that "we're standing on a turtle", and then eventually, through more investigation, "that turtle appears to be on another turtle", and so on.
We start with zero knowledge, an through repeated hypothesis testing, and null hypothesis falsifying, expand outward. We don't have to know how or why that first turtle got there, to justify that it exists. Those are independent questions that deserve investigations upon themselves.
This isn't subject to an infinite regress problem.
Likewise, when investigating the biological history of our lineage, we can figure out what is, and establish it as knowledge, without knowing what came beforehand.