I am not so sure that we need an escape route from subjectivism, so long as we can continue to show that attempts which offer possible defeaters to objective reality can also be offered their own set of defeaters. That is to say, the default view of reality should be the view that one is apt to hold his/her entire life absent from some philosophical objection that reality can not be known/justified in any objective sense. Of course, this does not leave one with a “proof” for objective reality. But, if opposing arguments can be found to be faulty (or especially self-defeating) then one is has rational justification to hold to the traditional/common sense view that one knows the outside world. That is to say, in the absence of an airtight, irrefutable argument that outside reality cannot be known, the fact that humans seem to experience outside reality (and this being an understatement) will always serve as a stronger “premise” within any argument (conscious or subconscious) for an external world. Only If one can provide reason that one should not necessarily (and the here the word is to be taken literally as in the philosophic sense) submit to the conclusions of any argument for subjectivity, then it is more than reasonable to continue to hold to the truth of an external, and knowable, world.
With that said, are there arguments given that provide a defeater to the belief in an external objective world? Of course. There are multiple arguments for such a view. However, it also seems that all of these arguments boil down to either one of two claims:
1) Due to our utter dependence on sense-data, perceptions, impressions, etc. we cannot know the external world.
2) Due to our utter dependence on sense-data, perceptions, impressions, etc. we are not justified in our belief of an external and/or objective world.
Notice that these are very similar conclusions, yet the second is more difficult to deal with, as its claim seems to be a little less strong.
The first claim is self-refuting. It cannot meet the requirements of its own claim. For instance, it the claim may be reworked to convey what it is actually saying. This simply means that for every negation, there must be an affirmation. The only way that one may claim number 1 above is if and only if one can know the external world. How does one know that one cannot know the external world, if in fact, no one can know anything about the external world? One must know something of the external world in order to know that the knowledge that I claim I have of an external world is not true of the external world. Or here is different angle by which to see that it is self-defeating. The statement could also be stated thus: ” We cannot know truths concerning an external world.” However, this is itself a truth claim about the external world. If it is true, then it is false, because one may now know a truth concerning the external world, namely, that one cannot know truths concerning the external world. On the other hand, if one can know truths of the external world, then surely there may other truths concerning the external world, as well. It would be Special Pleading to argue otherwise (not to mention one would have to know those ‘new’ truths they are arguing concerning the external world).
The second claim is more difficult. It seems to be more ‘humble’ in its approach. It does not say that one cannot know truths concerning the possibility of the external world; it simply says that, given the arguments for subjectivity, one is now not justified in holding to the view that truths can actually be known of the possibility of an external and objective world. However, this claim will also end up defeating itself. First, any argument given for the truthfulness of subjectivity of the external world must assume that there are external objects (one other being at the very least) that the arguments are true for. These would necessarily have to be true for all, objectively, if they are true at all. Of course, if the argument is true for all people, places, and time, (even if there were just one other) regardless of their own view, then this would be an objective truth and not subjective. (This is also exactly why a solipsist cannot argue another person into the solipsist camp, because obviously, if true, then there is no one else to argue into the view. However, if a ‘real’ person were argued into the camp, then it fails simply because there was another ‘real’ person.)
Also to say that ‘we do not have justification to hold to an external/objective world’ is simply tantamount to saying that, “We do not have any justification for holding any truths, or theories, of an external/objective reality.” Of course, this is self-defeating, as this itself must be a justified truth concerning external/objective reality. However, if it is, then one can obviously hold to justified truths concerning a possible external/objective reality. If true, then it is false, and on the other hand, if true, then the door is open to truths of the external world and one cannot arbitrarily shut the door to other possibilities concerning it.