If we are to assume that most of our brains and the five senses are functioning properly, thinking of nothing is impossible, for the very act of thinking involves assembling recollections of images and memories of what we have previously been exposed to. An individual may be thinking of something that is unimportant in the grand scheme of things, but he or she is still thinking. We even think while immersed in that state of unconsciousness known as sleep. These thoughts come to us in the form of dreams. We certainly cannot recall all of them. It has been suggested that if we could, it would render us insane. Dreams that occur closest to the time we awaken; however, tend to be remembered, and thus we think.
Is it possible to think of events that have not yet occurred? Of course it is, because our brains are capable of allowing us to look forward to any given event like an upcoming vacation trip or baseball game. In other instances, we can tune out our present surroundings and daydream about anything we so desire at any given time, such as during a boring lecture by some long-winded speaker. This is known as fantasizing. Depending on one's imaginative flair, we can create whatever scenario we wish with the power of our minds. Although we can formulate fantasies that can become quite incredible in scope, it is because we possess the ability to piece together any combination of imagery we have become familiar with throughout our lives in any arrangement we see fit.
It may not seem so, but even when we stare vacantly at nothing in particular, we are still thinking. We might not be concentrating on a specific reflection, but provided we are not blind and therefore unaccustomed to how the world around us should appear; the objects in our field of vision are nevertheless present. Therefore, if anything; we will think of whatever we see at the most subliminal level. For example, if we are riding as a passenger in a car traveling down the highway and our eyes become focused on something such as a long-abandoned, dilapidated barn situated in the middle of a pasture, we can very well find ourselves subconsciously recalling what we learned in school and begin to envision a long-dead farmer tending to his livestock or crops with period clothing and equipment from the 19th century. Such contemplation may only last for a fraction of a second, but the fact remains that we are still thinking. In a very short span of time, our collective imaginations may wander numerous times at the spectacle; however subtle, of a single image.
Even when we become so fatigued that we can barely keep our eyes open, we continue to think at a surprisingly fast rate of speed. This is but one reason why humans are the most intelligent species this world has to offer.
How we utilize this trait is another matter altogether.