What was the cause for the …

Causes of the French Revolution:

Despotic rule of Louis XVI: He became the ruler of France in 1774. He had drained the financial resources of France in wars. For the cost of his regular extravagant expenses, he increased taxes which were paid by the third estate.
Division of French society: The French society was divided into three estates; first, second and third estates, respectively. Among them, First and Second Estates had 10 per cent of the total population and were the clergy and nobles. They enjoyed all privileges. Rest of the 90 per cent was of the third estate, which paid all kinds of direct or indirect taxes. This kind of discrimination led to the revolution by the third estate.
Rising prices: The population of France had increased. This resulted into more demand of food grains. So the price of bread rose rapidly, which the poor could not afford to buy. So the gap between the rich and poor widened.
Inspiration of the philosophers: Philosophers like John Locke, Jean Jacques Rousseau and Montesquieu propagated the ideas of having a society where people would enjoy freedom, equal laws and equal opportunities. Their ideas inspired the people of France to realise their dreams.
Role of middle class: The 18th century witnessed the emergence of social groups termed as middle class, who earned their wealth through an expanding trade of the manufactured goods, being exported. There were other professionals also like lawyers, administrative officials who were all educated. They believed that no group in society should be privileged by birth.

Role of philoshopers:

(i) The philosophers played an important role in the French Revolution. They inspired the common mass of France with their revolutionary ideas and prepared them to fight against injustices. 
(ii) They did not believe in the doctrine of the divine and absolute right of the monarch. In his Two Treatises of Government John Locke refuted this doctrine strongly. 
(iii) Rousseau carried the idea forward proposing a form of government based on a social contract between people and their representatives. 
(iv) In The Spirit of the Laws, Montesquieu proposed a division of power within the government between the legislative, the executive and the judiciary. 
(v) The ideas of these philosophers were discussed intensively in salons and coffee-houses and spread among people through books and newspapers. These were frequently read aloud in groups for the benefit of those who were illiterate. Thus, the philosophers contributed a lot in bringing of the French Revolution.