Isaac Newton (1642–1727), in his 1687 work, Principia, placed physics understanding on a deeper level by deducing a law of gravity and three general laws of motion that apply to all objects: Newton's first law of motion states that an object remains at rest or continues in a state of uniform motion if no external force acts upon the object. Newton's second law of motion states that if a net force acts on an object, it will cause an acceleration of that object. Newton's third law of motion states that for every force there is an equal and opposite force. Therefore, if one object exerts a force on a second object, the second exerts an equal and oppositely directed force on the first one.