Abu Yusuf was chief jurist for Abbasid Caliph Harun al-Rashid, for whom he wrote the "Book of Taxation" ("Kitab al-Kharaj"). This book outlined Abu Yusuf's ideas on taxation, public finance, and agricultural production. He discussed proportional tax on produce instead of fixed taxes on property as being superior as an incentive to bring more land into cultivation. He also advocated forgiving tax policies which favor the producer and a centralized tax administration to reduce corruption. Abu Yusuf favored the use of tax revenues for socioeconomic infrastructure, and included discussion of various types of taxes, including sales tax, death taxes, and import tariffs. The power of supply and demand was understood to some extent by various early Muslim scholars as well. Ibn Taymiyyah illustrates:  Other important Muslim scholars who wrote about economics include al-Mawardi (1075–1158) and Ibn Taimiyah (1263–1328). Perhaps the most well known Islamic scholar who wrote about economics was Ibn Khaldun of Tunisia (1332–1406), considered a father of modern economics, Ibn Khaldun wrote on economic and political theory in the introduction, or "Muqaddimah" ("Prolegomena"), of his "History of the World" ("Kitab al-Ibar"). In the book, he discussed what he called "asabiyyah" (social cohesion), which he sourced as the cause of some civilizations becoming great and others not. Ibn Khaldun felt that many social forces are cyclic, although there can be sudden sharp turns that break the pattern. His idea about the benefits of the division of labor also relate to "asabiyya", the greater the social cohesion, the more complex the successful division may be, the greater the economic growth. He noted that growth and development positively stimulate both supply and demand, and that the forces of supply and demand are what determine the prices of goods. He also noted macroeconomic forces of population growth, human capital development, and technological developments effects on development. In fact, Ibn Khaldun thought that population growth was directly a function of wealth. Although he understood that money served as a standard of value, a medium of exchange, and a preserver of value, he did not realize that the value of gold and silver changed based on the forces of supply and demand. He also introduced the concept known as the Khaldun-Laffer Curve (the relationship between tax rates and tax revenue increases as tax rates increase for a while, but then the increases in tax rates begin to cause a decrease in tax revenues as the taxes impose too great a cost to producers in the economy).