I thought I would write a little something here to assist those that wish to flex their debate muscles. Some appear to be unacquainted with the construction of logical arguments, so it would behoove them to master some of those skills in order to be more effective in a competitive arena. Having studied both the Western Aristotelian discipline as well as the navya-nyAya school (contemporaneous with Mahaprabhu), I have some background to draw upon.
Let's start with the basic premise/conclusion format. 1) If A then B. 2) If B then C. 3) Therefore, if A then C. There is a special symbol for 'then', which is looks like the letter 'U' on its side. So we have two premises (1 & 2), which lead to a conclusion (3).
In order for any premise to be considered valid, there must be evidence (pramANa) to support it. For example: Wood coming into contact with flame will ignite. The type of evidence in this case is direct observation (pratyakSa, or sense perception).
One of the categories of pramANa is authority. That is where guru/sAdhu/zAstra comes in. What exactly does that mean? It means that if your guru teaches something that is in agreement with the scriptures and the consensus of other holy persons, then it should be accepted as authoritative. But what makes it viable in the first place? It is supposed to be based on knowledge gained through some channel. In other words, it is inherently dependent upon other types of pramANa, otherwise it would just be some empty dogma.
If someone argues that Srila Gaurapremanandeharibol Maharaj states "Blue fish are all Vaishnavas", therefore it must be true, because Maharaj is an empowered incarnation of Vishnu - then acceptance of that argument depends on the viability of the premise that he is such an incarnation. On its own it does not have any weight as a logical argument. It requires first establishing the viability of the premise, which may not be such an easy thing to prove.