Perhaps a good place to start would be to state what I don't believe: 

1. I do not believe in monsters, therefore I do not describe Bigfoot as such. 
2. I do not believe that there is anything paranormal about Bigfoot or any related animals. I do not believe that Bigfoot come from outer space or have flown on space ships. I do not believe that Bigfoot lives in another dimension, is invisible, or engages in "shape shifting" or anything like that. The jury is out on whether Elvis (Presley, Costello, or Stojko) has ever encountered Bigfoot in the vegetable aisle at the supermarket, but I'm just not going to go there.
3. I do not believe that the entire Bigfoot. phenomenon can be answered (and conveniently dismissed) by any one explanation, such as fake feet, misidentified animal footprint finds and sightings, a guy in a gorilla suit, etc... (see my page on why I find this interesting). And...
4. I don't believe in Bigfoot.

Hmm? Come again? I thought you had all this interest in Bigfoot, and now you're saying you don't believe in him? What gives? Here's the thing (Monk is such a cool show...) - that's right, I don't believe in "him", not when described like that. Him is a third person singular pronoun, implying that there is one, solitary, lonely, nearly omnipresent being who is responsible for every reported incidence of a Bigfoot sighting, track find, otherwise unidentifiable sound in the woods, mysterious putrid odor in uninhabited places in conjunction with other observed phenomena, and hair and scat finds which go to the lab and come back with the result of "unidentified mammal" which come from semi-inhabited and thoroughly remote locations all over the lower 48 states, in every Canadian province, and from Alaska, not to mention Asia, China, Russia, South America... Yes, that one Bigfoot must really get around!

I think that saying I "believe in Bigfoot" is kind of like saying I believe in Santa Claus or the Easter Bunny. The idea of a singular Bigfoot "monster" or whatever you want to call it is about as plausible, only without the chocolate or the presents. If you ask me if I'm a "believer" I'll say no, although if you're seriously interested in the subject I may qualify the question.

In fact, in the interest of clearing away the dumb stuff from the interesting stuff there are a number of descriptive words I'd like to strike from the Bigfoot vocabulary list :

• Believer - who knows what someone might be implying by saying you're one of these? The implication is that you've decided for certain that there is such a thing as Bigfoot, even though you may have never encountered one or had a strong reason to think that. Make no mistake, there are people who claim to have a very good reason to declare that Bigfoot(s) exist, but there are some of us who are interested but don't have grounds to make that declaration. Yet.
• Monster - also add Bogyman, Thing, Swamp Creature, whatever else under this heading. As I said, I don't believe in monsters...at least not the non-human kind. Monsters are supposed to be mean, whereas Bigfoots are not mean. Intimidating, yes. Mean/harmful/malevolent, no.
• Legendary - the implication is "a well-known story or series of stories that are common to a culture and not true". We don't know that the Bigfoot stories are not true. That's why this is interesting. 
• Myth - put "urban legend" in this one too. We don't know that it isn't true. See "Legendary".
• "On the prowl", "lurking", "stalking" - these are activities ascribed to creepy beings, so as to identify them as such. If there was a Bigfoot summer camp these items would surely be on the recreation agenda. I seriously doubt that any Bigfoot has any such notion as he steps out for the evening that he's going to see how scary he can be, perhaps with the exception of some of the observed territorial behaviors. More likely activities, which could be mistaken for the spooky ones mentioned above, would be "looking for food", "seeking shelter", and "assessing potential threats from weird little hairless cloth-covered apes, who seem to be lurking, stalking, and on the prowl in my forest".
• Abominable Snowman - 1) we have no reason to call any Bigfoot creatures abominable, 2) they don't live in, nor are they made out of, snow. Duh. Besides, the name came from an erroneous translation of the Tibetan language *.

I'm sure others will occur to me as I continue this - I'll edit accordingly.

So, what then do I believe, if I don't believe in Bigfoot? A few things:

1. I believe that there is enough evidence gathered that the question of the creatures' existence has merit, is interesting, and is worth pursuing. The first question to ask is not, "Does Bigfoot exist?", but rather, "Should we spend any time, resources, or brain-power asking whether Bigfoot exists?". 
2. I believe that if such a creature exists it is an animal species. This is the only answer that makes sense based on today's science.
3. It being an animal species, it's impossible that there is just one, or only a few. There must be a breeding population large enough to sustain the gene pool. Some Biologists say that could be done with a few hundred live animals - others say it would take several thousand in North America. Obviously, this is a point of some contention, and I'm not a Biologist, so I don't have a final pronouncement on how many Bigfoots it takes to procreate a species. Or screw in a light bulb.
4. Bigfoots are most likely apes and not men, despite an abundance of reported manlike traits. They are probably offshoots of the Asian Gigantopithecus apes - as far as I know this is the only scientifically verified species of ape that is thought to have walked upright. There are several similarities of the theories of what this ape looked like, how it moved, etc... to reports of Bigfoot sightings that one can draw a correlation.
5. The existence of the species will not likely be accepted by the mainstream scientific community nor the general public without a live or dead specimen, either whole or body parts, which at this time we don't have.

I maintain that there is enough physical and strong anecdotal evidence that has not sufficiently been debunked, despite numerous attempts, that there is a strong possibility that these creatures exist. Again, I will not declare myself a believer at this point. I will, however, reserve the right to change my mind on that. If I ever personally get a clear enough look at a Bigfoot to where I can completely rule out a misidentification or other misperception, or if I otherwise come across evidence that completely rules out other possibilities, at that point I'll declare that I believe in Bigfoot.

*Sanderson, Ivan T., "Abominable Snowmen, Legend Come to Life", 1968, Pyramid Books. pp 51-52.