In determining whether a creature could exist, the following method of analysis is used.
I. How reliable and unambiguous is the evidence?
Are there actually people who have claimed to have witnessed the creature first hand, or is it just a legend? What precisely does the report say? How reliable are the witnesses? Could they have been misled by false information? And if it is a Talmudic account – does the account mean that it is a real creature, or is it a metaphor? Is there any physical evidence, and could that evidence be interpreted in different ways?
II. What is the biological viability of such a creature?
Zoology is a highly developed science, and contemporary scientists are extremely knowledgeable about the natural world. Of course, there are wondrous new discoveries that are made every week. Nevertheless, we now possess a fairly good idea of which types of creatures are biologically viable, and one can now predict what type of wonders will and will not be discovered. For instance, we can expect to find new wondrous evidence of intelligence in certain insects. But we can be sure that no insects fifty feet in length will be discovered, for reasons that we shall later discuss.
However, sometimes people rule out the biological viability of a creature based on a popular conception of what the name of the creature refers to. Yet by slightly redefining the creature, it is often possible to conceive of a creature that is indeed biologically viable. For example, a mermaid that is half human being and half fish is biologically impossible, but a member of the seal or manatee family with more humanlike arms and facial features is certainly viable.
Some point out that even if the existence of such an animal contradicts the laws of science, it is possible that it exists by way of a supernatural miracle. This is indeed a possibility. Yet there are numerous classical authorities who state that positing the existence of miracles is a last resort and they are certainly not God’s preferred system for running the world. Accordingly, it is highly unlikely that there are creatures which can exist only through perpetual open miracles.
III. Can we satisfactorily account for the evidence if the creature is presumed not to exist?
It might seem easy for a skeptic to dismiss the possible existence of an extraordinary creature. But there is usually a considerable body of eyewitness testimony or other evidence of the creature’s existence that needs to be accounted for. As unlikely as something may be, a large body of eyewitness testimony from reliable witnesses, or other testimony from significant sources, may force one to accept the possibility of the existence of such a creature, or at the very least to account for the testimony in some way.
IV. Can we satisfactorily account for the lack of evidence if the creature is presumed to exist?
Proposing that a given creature does exist, on the other hand, leaves one with the problem of why there is no concrete evidence for it. The vast majority of creatures are known not from just one or two specimens but from many hundreds or thousands, both dead and alive. The total lack of any evidence for any fabulous creature is very revealing.
I leave it to the reader to apply this analysis to the Gemara's statement that bats lay eggs.