A valuable question to ask yourself, it can have unexpected consequences. As you ask this question, lovingly and carefully, you may discover that there is very little of which you can surely say, "I know that this is true."
Let yourself celebrate uncertainty when you find it in yourself. For each acknowledged uncertainty liberates you from a constraint you no longer need.
You may find you answer both 'yes' and 'no'. For instance, if you ask yourself, "Can I fly?" one clear truth is, "No, my physical body cannot fly unaided at the moment" and another clear truth may be, "Yes, my spirit flies freely with ease and joy".
Value such an answer for it reveals to you how rich and varied your experience is. Let uncertainty and multiple truths open your heart to the magnificence of your own being and the Universe in which you reside.