The Four Seals of the Dharma
As described in The Dharma Jewel, there are many different schools of Buddhism, which provide different presentations of the Buddha's teaching. Different schools appeal to different personalities, and no one school claims to be 'right'.
So what is it that all forms of Buddhism have in common, and what differentiates Buddhism from other religions and philosophies?
The defining features of Buddhism are THE FOUR SEALS OF THE DHARMA - four statements about the world which form the basis of all Buddhist teachings. The four seals aren't 'revealed truths' which we have to take on trust from some self-proclaimed 'prophet' who claims to have heard the voice of God, they are philosophical statements derived from logic and experience.
All phenomena are subject to change, growth, dissolution and decay. Even the sun, planets and galaxies are changing and will one day cease to exist.
(2) All phenomena are unfindable upon analysis.
If you search for the ultimate nature of your car, you won't find it. All you'll find is parts, the causes of those parts coming together, and mental projection or 'imputation' of something that performs the function of a car. Similarly with the ego - the self. The essential nature of the self is as unfindable as the essential nature of a car or Milinda's chariot. 'We are such stuff as dreams are made on'.
(3) Materialistic existence is ultimately unsatisfactory.
All emotions based on the three mental poisons of attachment, aversion and ignorance are ultimately painful. You can never have enough worldly possessions, and even if you did you'd worry about losing them since they are all impermanent. And you've got to lose the lot eventually when you shuffle off your coil. All materialistic cravings eventually lead to dissappointment and worse. This is known as dukkha.
(4) The true nature of mind is clarity and peace, 'but whilst this muddy vesture of decay doth grossly close it in' we cannot experience it. The sky-like clarity of the mind is obscured by the thunder clouds of anger, attachment and ignorance. The real nature of the mind is Nirvana - which is NOT nothingness, but the non-conceptual peace 'which passeth all understanding'.