The case of the missing umbrella:

The most universal definition of knowledge is that the latter is ‘justified true belief’, meaning that whatever information we think we possess must be corroborated by sufficient evidence but also, and just as importantly, by an inner conviction that, what we know, is actually the case. In our conscious life, we may, indeed, be fully aware of our home address or the year of the battle of Hastings. We simply know that we know such facts. However, our ordinary life is full of potential lapses of memory and Sigmund Freud first attempted to shed some light on what he called ‘the psychic mechanism of forgetfulness’, in a short essay published in 1898. Freud was interested in the unconscious, unfulfilled desires hidden behind such temporary dysfunctions. However, we are more concerned, here, with the philosophical effects of our occasional lapses of knowledge in everyday situations.
Let’s take the example of a missing umbrella. The weather has not been so good for several days and, for some reason, unknown to you, you haven’t taken your usual precaution of taking an umbrella while leaving the house. After several days, it gradually dawns on you that the reason for not using the umbrella was because you somehow knew that it was actually nowhere to be found. It is as if your unconscious mind had been informing you, in a mysterious way, that there was no need to take an umbrella. We have all experienced such a situation which reveals some fundamental aspects of our knowledge of both ‘Being’ and ‘Non-Being’. In his famous 1945 essay on ‘Being and Nothingness’, Jean-Paul Sartre highlights a key principle of our understanding of Being, by warning his reader that ‘the permanent possibility of non-being, outside us and within, conditions our questions about being.’ But under which circumstances can we ever claim to be conscious, and therefore to have any form of knowledge of so-called ‘nothingness’?
Sartre provides a telling example of such an occurrence in the opening pages of his long essay: he has an appointment with Pierre in a café, at four o’clock and arrives a quarter of an hour late. He is surprised not to see Pierre as he is always punctual. Sartre comments: ‘Is there an intuition of Pierre’s absence, or does negation, indeed, enter in only with judgement?’ In other words, do I experience the absence of Pierre or do I arrive at the conclusion that if Pierre is nowhere to be seen, he must, indeed, NOT be in the café. In fact, Pierre’s absence is not an evanescent, elusive event. It is fully accessible to my knowledge since through the presence to my mind of his physical absence, Pierre makes me aware of his former existence. The same conclusion would equally apply to the missing umbrella, which, through its physical absence eventually reminds me of its former existence and usefulness to me. It is out of their similar ‘nothingness’ that both Pierre and the umbrella eventually re-establish their presence in our mind.