Upon what was this sunny belief based, nearly a year ago? Or, to pose a different but perhaps better question: why did I believe it?
The first question assumes a particular view of how knowledge works, known as foundationalism. When wondering about the justifiability of a belief, the foundationalist wonders what other beliefs lie "beneath" it, giving it basis. If we find, at the bottom of the heap, a belief that stands on its own legs--such as a "self-evident" belief--then the believer is justified in holding that belief way up on top. (Whether that belief happens to be "true" is a related, but different, question from whether it is "justified.") So take this belief: "today is 21 October 2011." Holding it up are beliefs about one's grasp on the passage of days, and about the accuracy of the calendar. Supporting those are beliefs about the reliability of those in charge of determining what day and time it indeed is (I favor the National Institute of Standards and Technology time service hosted at the University of Colorado-Boulder), and of one's own senses...etc. If one eventually arrives at some self-justifying belief...well, then there you have it.
Foundationalism has issues. Is there really a bottom to the stack, or are we instead stuck with "turtles all the way down"? And one person's self-justifying belief ("I think therefore I am") is another's self-delusion (crankily proposed by dear Neitzsche in the opening salvos of a late work). Well, some philosophical problems are solved, and others are just gotten over, so let's get over this one and consider the second question-why did I believe a new barn would be sitting there come April? Did I believe it because I thought it justified? Or true? Or did I believe it for other reasons? If you'd found yourself in the clear cool light of a late November evening, auguring a new barn...why would you have done so?
Let's move on to this important fact: IT'S OCTOBER 2011 AND THERE IS NO BARN, either on that old foundation or elsewhere here at 149 Murray Road. So...what about that belief of 11 months ago? Would I have been better off without it?