You do not wash the cup, you wash the dirt; if you were to wash the cup, it would disappear completely. So, it is the dirt that is washable and has nothing at all to do with the cup.
This example is quite a good illustration of one of the most profound theories of the bodhisattvayana: we all have the potential to become Buddhas because we have Buddha nature. The problem is we have yet to realise it.

Dzongsar Khyentse Rinpoche

from the book Not for Happiness: A Guide to the So-Called Preliminary Practices

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Further quotes from the book Not for Happiness:

• Our fundamental problem
• We must also practice it
• Altruism bolsters self-confidence
• Nothing genuinely works in samsara
• Our most important companion
• Self Trapped
• Relative and absolute truth
• Being able to start practicing right away
• Obstacles Create Fertile Ground for Practice
• It cannot be fixed
• Life is a stream of sensory illusions
• Rip that ego apart
• Sign of a mature practitioner
• Not designed to cheer you up
• Three higher trainings
• Dawn of wisdom
• Avoid being distracted
• Without the personal advice of Buddha
• Mara’s five arrows
• Filtered perception