The topic of the VT debate was “Does God Probably Exist, or Not?” I used a cumulative case approach for my arguments. Below is my opening statement:
This evening we will be defending the proposition “God probably exists.” We will present three different arguments for theism. To defend our case we will be using the prime principle of confirmation: Whenever we are considering two competing hypotheses, an observation counts as evidence in favor of the hypothesis under which the observation has the highest probability. This principle is sound under all interpretations of probability. Each argument must be taken on its own grounds and one cannot arrive at “God” at the end of each argument. The conjunction of arguments is what is needed to make a cumulative case for the existence of God.
The Likelihood Principle of Confirmation theory states as follows. Let h1 and h2 be two be competing hypothesis (in this case the existence of X and ~X, with X being a first cause, fine-tuner, etc.). According to the Likelihood Principle, an observation e counts as evidence in favor of hypothesis h1 over h2 if the observation is more probable under h1 than h2. Thus, e counts in favor of h1 over h2 if P(e|h1) > P(e|h2), where P(e|h1) and P(e|h2) depict a conditional probability of e on h1 and h2, respectively. The degree to which the evidence counts in favor of one hypothesis over another is proportional to the degree to which e is more probable under h1 than h2: particularly, it is proportional to P(e|h1)/P(e|h2) . The Likelihood Principle seems to be sound under all interpretations of probability. This form is concerned with epistemic probability.
The Likelihood Principle can be derived from the so-called odds form of Bayes’ Theorem, which also allows one to give a precise statement to the degree to which evidence counts in favor of one hypothesis over another. The odds form of Bayes’ Theorem is P(h1|e)/P(h2|e) = [P(h1)/P(h2)] x [P(e|h1)/P(e|h2)]. The Likelihood Principle, however, does not require the applicability or truth of Bayes’ Theorem and can be given independent justification by appeal to our normal epistemic practices. 
I applied this principle to each of my arguments:
The Thomistic Cosmological Argument
- What we observe and experience in our universe is contingent.
- A network of causally dependent contingent things cannot be infinite.
- A network of causally dependent contingent things must be finite.
- Therefore, There must be a first cause in the network of contingent causes.
The Fine-Tuning Argument 
- Given the fine-tuning evidence, a life permitting universe/multiverse (LPM) is very, very epistemically unlikely under the non-existence of a fine-tuner (~FT): that is, P(LPM|~FT & k’) ≪ 1.
- Given the fine-tuning evidence, LPM is not unlikely under FT (Fine-Tuner): that is, ~P(LPM|FT & k’) ≪ 1.
- Therefore, LPM strongly supports FT over ~FT.
*Remember, k’ represents some appropriately chosen background information that does not include other arguments for the existence of God while merely k would encompass all background information, which would include the other arguments, and ≪ represents much, much less than (thus, making P(LPM|~FT & k’) close to zero).
The Moral Argument
- There are objective axiomatic/moral facts that obtain.
- Either the world alone or the world and a perfectly moral person best explain these facts.
- It is the case that the world and a perfectly moral person best explain these facts.
- Therefore, the world and a perfectly moral person best explain these facts.
So, what about the problem of dwindling probabilities in a cumulative case? When combining probabilities the end product is smaller (.5 multiplied by .5 = .25). This issue concerns the restricted conjunction rule for probability: P (A and B) = P (A) x P (B) (when A and B are independent). It also appears in the general conjunction rule of probability: P (A and B) = P (A) x P (B given A). When using a probability calculus the only time you would add probabilities is in disjunctive calculus (.25+ .25 = .5). This occurs in the restricted disjunction rule for probability and the general disjunction rule for probability, respectively: P (A or B) = P (A) + P (B) (when A and B are mutually exclusive), and P (A or B) = P (A)+ P (B) – P (A and B). Now, in order to avoid the problem of dwindling probabilities the conjunction of arguments must be used as one probability calculus. Even if these arguments weren’t used in the cumulative case form the converse probabilities would make the probability for the non-existence of God congruently smaller.
My closing remarks after my arguments were:
Remember from the beginning that we are using an epistemic probability calculus. Each argument must be evaluated on its own grounds with the selected background information k’, that is, no other arguments influencing the argument. Now, when we use these arguments in a cumulative case then we are left with: a spaceless, timeless, transcendent, uncaused, very intelligent, morally perfect person. This is sufficient for theism. Each of these evidences are much, much more likely under the hypothesis that God exists and very, very unlikely under the hypothesis that God does not exist. Given the total background knowledge k, ceteris paribus, all things being equal, it is more probable that God exists than not.
In the end, I believe I used a well-reasoned and sound modus operandi and approach for the debate.
 Robin Collins, “The Teleological Argument,” in The Blackwell Companion to Natural Theology Eds. William Lane Craig and J.P. Moreland (Oxford, UK: Blackwell, 2009), 205.
 Ibid., 207.