How is necessity used to justify rationalistic speculation? This is perhaps best explained by necessity's most persuasive advocate, Brand Blanshard, who defined reason as "the discovery of necessary connections":
The office of reason as it works in each of us is thus to construct, or reconstruct, the rational whole. The world of common sense is the result of a long attempt at such reconstruction. The world of the sciences is the result of the same attempt, carried out more critically and resolutely. The reason at work in philosophical speculation is not something different from that of these disciplines; it is the same, but operating under a more urgent feeling for what integrated knowledge demands. The most obvious of its demands is for consistency... [Reason and Analysis, 91, italics added]
Blanshard is essentially arguing that the reason at work in the sciences is the same as the reason at work in philosophical (i.e., rationalistic) speculation. It's the same because all these forms of reason are attempting to discover "necessary connections." The world, according to Blanshard, "is shot through with filaments of necessity." This means the world is a rational, intelligible order. It also means, at least by implication, that if you can grasp one necessary connection, you can reason yourself to others. If I already have two dollars in my pocket and I add another dollar, mustn't I have three dollars in my pocket? As a matter of logic applied to an ideal situation, yes; but as a matter of fact, no. What if I have a hole in my pocket? Then I might have only two dollars or one dollar. What is true by necessity for thought is not necessarily true empirically of the material world. Truth is fundamentally factual. Facts do not derive their truth from logical tropes or ideal necessities. While reality must have some form or nature, there is no necessity that it should have the particular form that it actually adopts. That the speed of light travels at 186,000 miles per second is a fact. There is no logical necessity in it. If a fact were logically necessary, it would be true, and ipso facto discoverable, a priori. But there are no a priori facts. Facts are empirical through and through. Facts are also surds, inexplicable in and of themselves (i.e., there are no ultimate explanations for them). They are cosmic accidents.
While Objectivists don't openly argue for necessity in order to justify rationalistic speculation, it's obviously a doctrine that appeals to their zeal for certainty. "Truths about ... facts are learned and validated by ... observation; and, qua truths, ... are ... necessary," writes Peikoff, adding "all truths are [necessary]." Peikoff's main argument for this position seems to be merely that "a true proposition must describe facts as they are."  However, here Peikoff seems to be confusing truth with necessity. What does it mean to say that a truth is "necessary"? In philosophy, it usually means, "it couldn't be any other way." The problem is: how can we know it couldn't be any other way? If there are no ultimate explanations (and there isn't: even God's existence wouldn't constitute an ultimate explanation), then how can we know that things could not have been different? After all, we assume things could be different all the time. Objectivists admit that there is a sort of contigency in facts when it comes to human volition; but they insist that no metaphysical fact could be different. To think otherwise, Objectivists seem to imply, opens the door to wishful thinking. But is this really true? Per usual, Objectivists provide no particular examples.
It could be argued that we don't really know whether facts are ultimately contingent or necessary. Even so, on pragmatic grounds, the contingency view appears more useful, at least for science and open-minded inquiry. It opens the door to counter-factuals, which, oddly enough, Objectivists often have trouble accepting. As Rob Bass noted:
Objectivists seem allergic to considering hypothetical or counterfactual cases. They are apt to accuse anyone who employs arguments involving counterfactuals of being “rationalistic" or “not grounded in reality." Apparently, they think this charge, which is usually unexplained, is sufficient to get them off the hook of having to actually address those arguments. The arguments are dismissed as arbitrary or meaningless or some such thing. Oddly, I find little warrant for this particular bit of lunacy in Rand’s own writings. (Is she to be accused of rationalism when she talks about indestructible robots?) Nonetheless, the attitude is quite common among Objectivists.
I could speculate that this is defensive maneuver to keep from having to deal with hard questions that Objectivism is ill-equipped to handle, but, for now, I will place little weight upon that speculation. I will confine myself to pointing out that reference to counterfactuals is nearly unavoidable if you want to think clearly about issues in philosophy of science such as causation and laws of nature. The claim, for example, that it is a law of nature that (say) unsupported rocks fall to the ground does not just report a regularity in the behavior of rocks. It also implies things like “if this rock were unsupported (which it isn’t), then it would fall ? – and that can be true even if the rock never is unsupported. Note that this is not just a material conditional (which is always true when the antecedent – the if-clause – is false) because we also want to deny claims like “if this rock were unsupported (which it isn’t), then it would turn into a bird and fly away."
A very similar point can be made about causation. To say that an event, A, causes another event, B, is not just to report that A is followed by B, but also implies (if there are no other causes of B or other possibilities for the occurrence of B about) that if A had not occurred, B would not have occurred.
Whether we speak of laws of nature or of causation, an adequate understanding of what is meant is not possible without reference to situations that do not actually occur – that is, without reference to counterfactuals. If we refuse to deal with counterfactuals, then the most that we will be able to manage in this area is some kind of Humean regularity analysis of causation and of laws of nature.
Once we admit counterfactuals, as we must to address important issues in the philosophy of science, it will be much harder for Objectivists to dismiss other arguments as merely hypothetical or rationalistic word-spinning. They will have to offer specific reasons that particular arguments that appeal to counterfactual states of affairs go wrong (if they do). General objections to counterfactual arguments won’t do the job.
Although the Objectivist "allergy" to counter-factuals is, as Bass speculates, most likely rooted in a desire to evade difficult questions which Objectivism is ill-equipped to answer, the belief in necessary facts essentially rationalizes this evasion and hardens it. The Objectivist allegation that counter-factuals are "rationalistic" involves a misunderstanding of rationalism and empiricism. Philosophers like Hume don't criticize rationalistic speculation because its rationalistic or speculative; they criticize it because it was often used to establish matters of fact. There's nothing wrong with rationalistic speculation, provided it is recognized as speculation, rather than fact. Speculations can be the source of rich hypotheses that can form the basis of various research programs and scientific experiments. After all, Einstien's theory of relativity had it's origin in speculative thought; but, and this is the critical point, it didn't end merely in speculation: on the contrary, it became the basis of experiments which corroborated the veracity of its claims. Philosophers may speculate all they like; what they shouldn't do is claim their speculations are true because they are "rational" or based on "necessary" truths. There are no such necessary truths which can form the basis of true speculations about matters of fact.
Objectivists are fond of Rand's "stolen concept" fallacy to refute views they disagree with. Couldn't Objectivists argue for necessity on the grounds that all denials of necessity must assume necessity? I'll examine this in my next post.