How can you say something is applicable to daily life?

You can’t always. The only defense is that it never hurts to have extra knowledge.

If what you’re looking for is alternate phrasing, try these:

it’s relevant to everyday life
it works in daily life
it pertains to ordinary life
it has practical everyday application
it’s useful on a daily basis

If you’re writing a paper, let’s say, and making an argument that some ancient wisdom is still worth studying today, I think you’re right to try to avoid clinical language such as “applicable” and phrase it more in everyday terms.

You might also come at it a different way and say something like “As long as human nature remains constant, ancient truths will have a bearing on our understanding of life.”

Does that help?

A more specific question might elicit a more specific answer.

“relevant to today”

Sorry. I totally misread the question. It was the first thing I looked at when I got up.

What ancient theory are you referring to? For example, Rome’s ancient government is what our current government in the US is based on, so it’s very relevant to today’s society. So are the reasons it fell.

You could word it like this, “This ancient theory is still relevant in today’s world.”

Just substitute the actual theory or concept (such as Democracy, or marriage) and then give examples for why the theory is still relevant, or why the theory still works.

Applicable theory sounds right. To prove your point, show something that we do differently due to this theory. If no such thing exists then obviously the theory is not applicable.