Final answer:
Considering the provided evidence and the reliability of the uncle's claim about seeing a ghost, we should take into account the uncle's fallibility and alternative explanations for his sighting, which suggests that the subjective probability that ghosts exist should not significantly increase.
Step-by-step explanation:
When initially presented with the statement that there is a 5% chance that ghosts exist, one might be skeptical. However, when considering a new piece of anecdotal evidence, such as the claim from a relative who has seen a ghost, one must also weigh the reliability of the source and the context.
Even though your uncle, a trusted source, asserts seeing a ghost, his poor eyesight and previous misconceptions, such as mistaking a bag of mulch for a dog, indicate that he is a fallible source. When evaluating the probability of ghosts existing, rather than simply updating the probability based on the new claim, one should account for these factors that could potentially undermine the credibility of the evidence presented.
In the realm of statistics and probability, it's essential to consider alternative explanations and contextual factors that could explain an observation. For example, given your uncle's poor eyesight, it is possible that he was mistaken in what he believed he saw. Therefore, while it might affect your degree of belief to some extent, it would not necessarily lead to a significant increase in the subjective probability that ghosts exist, due to the fallibility of the source and alternative explanations that could account for his experience.