Doubting the Claim of 57 Alien Species Catalogued
Are we alone in the extent of the known Universe? I believe it's a 'No'.
Our Sun is just another star within our galaxy; our galaxy is just another one within the Local Group of galaxies, and so on.
There is a very high possibility of other terrestrial life forms found in the Universe since there are many other stars that are similar in size and sequence (star's lifetime) to our Earth.
Although it would prove my belief true, I choose to doubt the claim of 57 alien species catalogued because of the following reasons:
The nearest star to our Earth is at least 4 light years away. This is a very far distance, and if the upper limit of traveling speed is light's speed, the aliens would have to travel at least 4 Earth years to reach Earth. If they were capturing solar or radiation en route as their vehicles' energy source, these vehicles would most likely appear black in order to absorb radiation more effectively. Also, to withstand high temperatures, speeds and probable impact, the material must be robust and may make the alien craft so big that is difficult to avoid detection.
HIGHER ORDER LIFE FORMS
If these aliens are able to travel here, they should be higher order life forms as compared to life forms on Earth. They must have studied Earth for long enough time before embarking here. Ability to sense and avoid danger are traits that must be present; it is less likely they will crash into Earth surface if they had avoided crashing into outer planets like Jupiter and Saturn which are more massive (hence more gravitation) before coming to Earth.
As mentioned earlier, ability to sense and avoid danger must be traits of higher order life forms, and like we humans, before sending humans onto the moon, we had sent probes before we talk about human landing. Similarly, we would find alien-deployed Earth probes but why there is no crashed Earth probes discovered and catalogued?
If there are 57 alien species catalogued, there must be extra-terrestrial visits exceeding this number, with some of these visits go unnoticed or occur at some other parts of the Earth and got covered up. Since aliens have visited Earth, it is very likely they might visit our Moon for information collection. If the aliens had sent a probe or unmanned vehicle onto Moon, they would leave a trail on the Moon's surface. With no activity on the Moon to erode evidence of alien's visit, the trails can stay for as long as the footprints left by the Moon's astronauts.
TRAITS OF HIGHER ORDER LIFE FORMS
Just like we are curious to find out the existence of other life forms other than ourselves in the Universe, aliens are similarly curious as well. The first 11 US unmanned missions to the Moon were unsuccessful, with some missing the Moon or landed on Moon but failed to transmit any data. Likewise, there would be several unsuccessful attempts from these extra-terrestrial life forms.
If aliens really do come to our planet, it is either those aliens are at the stage where they make unsuccessful attempts and got discovered by us or they are so advanced that there is absolutely no way we can detect their presence on Earth. But according to Sergeant Clifford Stone, there are 57 alien species catalogued, which mean we humans must be able to see or detect their presence in order to do so.
WHAT ENCOURAGES THEM HERE?
To explore the Universe? To conquer Earth? To relay us a message about our Universe? To spread their religious faith? Or they are looking for a better place to live in? There are far too many possibilities to guess.
If our moon is inhibited by some other life forms, human landing on the Moon might not have happened so soon, we would not know the reactions from inhibitants of the Moon. Fearing that would cause a rivalry between inhibitants of the Moon and Earth, more work has to be done before paying the Moon a visit. Likewise, the aliens would have the same concerns about coming to Earth, unless they are confident of success of their visits.
Sometimes we jump into believing the claims just because there was a play of words. Labelling as "Secrets kept from public by NASA" doesn't mean "Truth", things are sometimes kept as secrets because they may be misleading the people. Also, information that is "legally seminated under the FOIA (Freedom of Information Act)" doesn't mean disclosure of TRUE previously unreleased information. Note the use of the word "legally", bear in mind that as long an action breaks no law, it can be said to be "legal" under all Acts and Statutes.
I have taken a logical approach in looking at the claim; I'd be thrilled to get a confirmation on the existence of extra-terrestrial life forms but there are certainly many loopholes and unexplained blanks in Sergeant Clifford Stone's claim. Until his claims are supported with tangible evidence, I choose to doubt the credibility of his claim. "Absence of evidence is not evidence of absence", but no tangible evidence of presence does not amount to confirmation of presence.