Again, I can't help but feel like I've left the house with the stove on -- that something has been left undefined. We probably will need to circle back to carefully define other terms in our foundation, but, for now, we'll continue building up.
The next topic to cover, the first big step, is natural theology.
Fr. Hardon's Modern Catholic Dictionary defines natural theology as "(t)he knowledge of God, his existence and attributes,
derived from the world of nature and by the light of natural reason." Merriam-Webster concurs, giving the following definition: "theology deriving its knowledge of God from the study of nature independent of special revelation".
In doing natural theology we're drawing conclusions about God from only the natural world: the physical world around us, the laws of nature, ourselves and our own experiences. This makes the best foundational step, after defining terms, because its the least costly. It assumes the least.
We're taking as assumed no revelation, such as Sacred Scripture, but only the physical world around us and our own experiences. It's not "simpler" to start without these basic assumptions, though some have tried denying the existence of other minds, or any minds, or even physical reality. If other people aren't real or the world isn't real, then there is no argument from those imaginary people or that fictional world that you'd have to accept. Either position leaves you stuck, unable to be certain of anything. This goes against our every day experience and common sense. In order to move forward - and to satisfy this common sense - we'll accept those very basic premises and works from there.
The first example we'll look at is a personal favorite--one that had a profound effect on me: the Argument from Desire put forth by Christian apologist and author C. S. Lewis.
Hardon, John. 1999. "Natural Theology". In The Modern Catholic Dictionary. Inter Mirifica
Merriam-Webster's Dictionary. "Natural Theology". Online. http://www.merriam-webster.com/dictionary/natural%20theology