A synthetic proposition is a statement whose truth depends on facts. The predicate is not contained in the subject. All bachelors are unhappy is synthetic. The concept bachelor does not include unhappy. You need evidence to know if it is true. The statement adds information about the world. It is not true by definition.

The term comes from Immanuel Kant. He distinguished synthetic from analytic propositions. Analytic propositions are true by meaning. Synthetic propositions are true by fact. All bachelors are unmarried is analytic. All bachelors are unhappy is synthetic. The first is true by definition. The second requires observation.

Synthetic proposition examples

• The cat is on the mat.
• Water boils at 100 degrees Celsius.
• Some swans are black.
• Paris is the capital of France.
• Grass is green.

Synthetic propositions are informative. They tell you something you did not know. They are contingent. They could be false. The cat could be on the floor. Water could boil at a different temperature on another planet. Black swans exist, but they might not have. Synthetic propositions are the stuff of science and everyday knowledge. They are tested against experience. They are revised when new evidence emerges. They are not certain. They are probable. That is the trade-off. Analytic propositions are certain but uninformative. Synthetic propositions are informative but uncertain. Kant argued that some propositions are both synthetic and a priori. He called them synthetic a priori. Examples include mathematical truths and causal principles. That is controversial. Most philosophers accept the analytic-synthetic distinction. But the boundary is debated. Is water is H2O analytic or synthetic? The debate continues.