I've been conflicted about whether or not 9/11 was a government conspiracy for quite some time. For what it's worth, let me give a run-down of my position.
Before I spell out my position, allow me to make some brief comments on probability and induction (reasoning from specifics to generalities).
First, consider the idea of independence in probabilistic events. If there is a 0.01% chance that your tire will blow out while driving down the highway and a 0.5% chance that you will sneeze at any given time, then the chances that you will sneeze at exactly the same time as your tire blows out while driving down the highway is simply 0.01% x 0.5% = 0.00005%. As you add more simultaneous - but unrelated events - to the mix (say, the probability of seeing a green car, sneezing and blowing a tire all at the same time), the probability approaches zero exponentially quickly.
Two events are dependent if they are not independent. Another way to say this is that if two events occur simultaneously more frequently than their individual probabilities multiplied together, we can know they are not independent events. For example, there is some probability, let's say 1%, of driving over a pothole. If there is a 0.01% probability of blowing a tire but blowing a tire while driving over a pothole occurs 0.0002% (twice 0.01% x 1%), then we know that driving over a pothole and blowing a tire are not independent events. Of course, we have to be careful about drawing conclusions about causation because dependence between events A and B does not imply one caused the other... both could be the result of some common cause C.
This brings me to induction or reasoning from specifics to generalities. In general, when two or more individually improbable events occur simultaneously, we consider this remarkable. For example, if you got struck by lightning and bought the winning Powerball ticket on the same day, that would be an astoundingly remarkable coincidence because each of those events is individually vastly improbable. Now, humans are prone to fallacies in reasoning about probabilities, so we often commit the fallacy of assigning dependence between such improbable events because it seems that each is so improbable on its own that it is impossible (except through some miracle or magic or whatever) that they could both happen simultaneously. Many parlor tricks and card tricks exploit this weakness (e.g. the Birthday "Paradox") to amaze people. Remarkability, however, is the red flag that signals the brain to start looking for a pattern - if two or more individually improbable things happen together, it's probably a good idea to start looking for the causes. The evolutionary basis for this should be obvious - if you hear a stick crack in the forest, that may or may not be remarkable in itself... maybe the wind finally overpowered a dead branch. If you hear a bird flutter off suddenly, that also may not be remarkable... perhaps it finally noticed your presence and fled. But if you hear a stick crack and about the same time hear a bird flutter off suddenly, you better get your knife out and start looking for an exit (or prepare to stand your ground).
Coincidences may be nothing more than two unlikely things happening at the same time. But the more coincidences there are, the less likely this is the case. Casinos do not believe in coincidence. If you win a large sum against the house at table A and then win another large sum against the house at table B, you are likely to find yourself gently escorted from the premises, even if you did nothing wrong. But if you win large sums at tables A, B, C and D before the casino catches on to you, they won't be escorting you out, they'll be asking you to stay while they call the police.
I want to make a brief note on Ockham's razor. Ockham's razor basically states that a simpler explanation that explains all the facts is to be preferred to a more complex explanation. This is because it is more likely to be true by virtue of its simplicity. Let's say we have to competing hypotheses for a particular phenomenon. The first hypothesis creates a model with five irreducible objects in it. The second hypothesis creates a model with three irreducible objects. The second hypothesis is to be preferred because it is more probable that fewer things caused an event than many things. There are more degrees of freedom or ways to arrange five things than three things so it is more probable that an event is the result of the interaction of three things than of five.
The single most disturbing element of a variety of terrorist attacks - the Alfred P. Murrah building, the '93 WTC bombing, the 9/11 attacks, the 7/7 London bombing and the Madrid bombing are the occurrence of similar circumstances between these attacks that should have been all but completely independent. For example, on 9/11, NORAD was running exercises involving hijacked planes. This contributed to the confusion during the response to the real hijackings. That, in itself, is a coincidence and somewhat remarkable. In the 7/7 London bombing, again, there was a terrorism-response training exercise going on at the very subway station where the bombing occurred. Now, there is a stick cracking and a bird is fluttering off suddenly and my curiosity is piqued. There are other coincidences between these attacks that I don't have the time or inclination to go into here but which I find alarming.
There were a lot of ATF documents related to the raids at Waco and Ruby Ridge stored at the Alfred P. Murrah building that were destroyed during that explosion. No BATF personnel were killed. There were a lot of SEC files that were destroyed during the collapse of WTC 7, some estimates say 3,000 to 4,000 active cases were affected. No one died in that collapse.
Let's assume for the moment that the conspiracy theorists are right and the government did cause 9/11. There are several possibilities of the form a government conspiracy to engineer the events of 9/11 might take:
1) The DoD built special remote control military planes (and failed to even go to the effort to make them the right shape and paint them with the colors of the commercial flights they were supposed to be) and fired a cruise missile at the Pentagon. The people who boarded the flights are still alive somewhere, etc.
2) The CIA and/or DoD secretly funded, trained and groomed Osama bin Laden who, along with al-Zarqawi and Kaleed Sheikh Mohammed planned, funded and executed the 9/11 attacks
3) An extra-governmental secret society with high-ranked rogue agents placed throughout the political and national security apparatus and elsewhere planned, funded and executed the 9/11 attacks.
Possibility 1 is laughable and, unfortunately, constitutes a large portion of the 9/11 Truth nonsense.
Possibility 2 is more serious, but ultimately incredible - it would entail so many moving parts and would be so vulnerable to whistle-blowing, it would never hold up without ultimately being exposed.
Possibility 3 is credible but, if true, almost certainly not provable or supportable by any evidence.
And this is where I am at, currently. I see the motive for the state to create and propagate terrorism to be overriding and the motives of Islamic terrorists to crash planes into the WTC towers to be less than convincing. But then, it is not the state itself that would be the telic actor in the most credible scenario, 3. However, a secret society forming a proto-international government might have similar motives to the state. I see patterns (someday when I have some time, I'll go back over the info and try to organize these for presentation) in the various events, but absolutely no convincing, damning evidence, certainly nothing that goes beyond reasonable doubt. If there were such evidence, a criminal investigation and trial would have already occurred, I'm sure.
I can perceive patterns that seem beyond coincidence, I can see the motives of the state or even rogue actors within the state to covertly engineer terrorism but the more likely an explanation of the terrorist attacks is, the less probable any evidence could ever prove it (how do you connect the dots between Timothy McVeigh and Osama bin Laden, assuming there was a secret, rogue cabal within government behind both attacks?) Going back to Ockham's Razor: a secret rogue cabal within government is more credible than a vast, globe-arching Islamic terrorist network and interstate white extremist terrorist network. The government's theory has more moving parts than the rogue agent theory and both would explain the same events. So, the conundrum is this: The more probable the rogue agent theory (the fewer the moving parts) the less likely any evidence could be uncovered to show this to be the case exactly because it has fewer moving parts.
If 9/11 is the result of a secret rogue cabal within government, then the government's account of 9/11 is at least genuine and maybe even largely true. For example, I find any theories of the planes being remote piloted, etc. to be simply absurd - which implies that there really were suicidal hijackers on the planes. But that doesn't mean that they could not have been ultimately funded and directed by a rogue element within our government (and other governments). Also, I'm genuinely conflicted on the credibility of the collapse of WTC 7 but can't imagine how anyone could pull off loading it with explosives. In other words, possibility 3 begins to become unfalsifiable and consistent with the mainstream theory.
The difference between applying Ockham's Razor to nature and to conspiracy theories is that nature is not attempting to actively conceal its secrets and frustrate attempts to discover its underlying workings. Conspirators, on the other hand, are. So, the more probable a conspiracy theory is, the less likely any evidence will support it.
That's my take on 9/11. In my mind, it is very possible that the actions of those within government either actively or passively contributed to the events of 9/11 (among other terror events). But at the same time, the more likely a conspiracy theory is, the less evidence there is which can be brought to bear to support that theory (and exclude other theories), that is, the more unfalsifiable it becomes. Nevertheless, it is not outside the realm of possibility and certainly not "beyond them", there is no a priori reason that terrorists must wear robes and read the Q'ran rather than wear suits and ties and read the Wall Street Journal.