Clairvoyance is often an over-rated and romanticised ability, typified by august visions or eerie crystal balls. While it is true that both of these are included under the clairvoyant banner, they are by no means the bulk of it.
'Clair-voyance' is literally "clear-seeing", and denotes being able to see at a range beyond what is considered the normal human spectrum. To relate how this ties in with visions and spirits, a basic understanding of energy and vibrational patterns is necessary.
All things at their base forms are made of energy, this has been proven by science. How they interact with each other, and what vibrational level they operate on determines how we perceive them, eg. A chair as opposed to an apple. Objects with a relatively slow vibration rate will appear more solid and anchored in the physical world. Things of a higher vibrational level, however, tend to be more ethereal, elusive, and even to the point of invisibility. A good example that everyone would be familiar with is the differences between ice, water, and vapour - in which case, differing temperatures act as the trigger for a change in the vibration levels.
Everything we see can be accorded a different vibration rate which, for the vast majority, will fit in under the colour spectrum visible to the human eye - red being the lowest, and indigo the highest. But beyond that lies the realm of clairvoyance, where the boundary is not set at the usual human limit.
Of the great variety of things that a clairvoyant person may see, spirits are perhaps the most publicised. They are, by their very nature, without physical form and therefore existing on a slightly higher frequency than we are - the degree of vibration depending on the type of spirit itself. Clairvoyants may also see events, either past, present, or future, and there has been a popular sub-branch of clairvoyance that has sprung up and is based entirely on this, called psychometry. It has often been asked why such visions or foreknowledge of events is most usually of a bad or tragic nature, for example, a plane crash. The answer is surprisingly simple, given what is known of frequency rates: negative events act at a lower vibrational level, in a sense 'shouting' out to the clairvoyant and making them much easier to tune into, while positive actions have a much more subtle and higher frequency.
The techniques for seeing such things are varied and probably individual for each person, although there are certain elements or methods that are considered useful by large amounts of people. The stereotyped 'crystal ball' is not as silly as it's made out to be, as it allows the clairvoyant to relax their eyes and enter into a light state of meditation - something particularly useful for a budding clairvoyant to hone their skills. Of course, a crystal ball is not the only form of this method, only the most sensationalised and arguably the most expensive. Other similar techniques include using a bowl filled with black ink, water with crystals at the bottom, and I have even heard of a successful attempt using a mug full of coffee. Other popular triggers are items with existing magical associations, such as tarot cards, runes, and other divination tools. More often than not, the form of clairvoyance that this initiates is primarily seen within the mind rather than a seemingl! y physical image or vision, which brings us to the next main variation in clairvoyance - how it is seen.
For simplicity's sake, I'll call the two main forms of clairvoyant sight 'physical' and 'non-physical'. Physical sight should not be confused with seeing something that is physically there, but rather having it appear in usual surroundings, no matter how incongruous it may be. A perfect, if extreme example of this would be what the little boy saw in the movie, "The Sixth Sense" (which, incidentally, had a great deal of truth in it, despite Hollywood's influence). In this movie, the boy randomly saw the spirits of dead people appearing as solid as you or I. This is perhaps the most extravagant case of clairvoyance, and on the whole it is not so defined. Have you ever been convinced you have seen movement from the corner of your eye, then to have found nothing when you turn to face it? Most likely, you will have dismissed such happenings as imagination or nerves. Well, perhaps next time you should rethink that, given that the rods (outer sides) of our eyes are much more sensitive than the cones (center). We use our cones everyday whenever we look directly at something, and it is this extended use that has damaged them and made them less likely to pick up the finer details. The rods, on the other hand, are in much better condition, and will in essence, see more. A simple experiment to test the authenticity of this is to look up at the night sky. The stars will always appear brighter and sharper when we are not looking at them directly.
The second form, the non-physical, is in my experience the most common. Often experienced as a picture in your mind's eye, or as an image projected on the back of your eyelids (when your eyes are closed), it's much harder to pin down, and therefore is often discounted as imagination by beginning clairvoyants. It is, however, every bit as valid as seeing things physically, and in fact can often be more useful to an experienced clairvoyant as it's usually accompanied by clairsentience. It takes a while to hone and trust this form, but for goodness sakes don't discount it. It's recommended that someone wishing to extend their clairvoyant abilities keep a journal of all they see and any resulting impressions, not matter how odd they may seem at the time. I can vouch for this personally, as it's often an eye-opening experience to go back later and have details verified.
So, you're probably asking, what causes clairvoyance and can anyone learn to do it? To answer the latter, yes, anyone can everyone has the capabilities to see clairvoyantly, given a little time, practice and patience. As for what causes it, it has been theorised that humans are slowly evolving into greater and greater clairvoyant abilities, and those that already use such techniques have just jumped slightly ahead on the evolutionary ladder. This view is supported by several Greek philosophers of Classical times referring to the "tri-coloured rainbow". So, if we've increased from three to seven in just a few thousand years, imagine what the future holds