Ayatullah Sistani and Ayatullah Khoei and Ayatullah Sayyed Muhammad Baqir al-Sadr and most of our great scholars say that playing chess is forbidden, based on very clear evidence from the Prophet (SAWA) and Ahlul Bayt (AS). In Wasaa'il al-Shia , Volume 17, page 323 , you can see at least eight authentic Hadeeths stating that chess is forbidden,bwhile no single Hadeeth allowing chess. Some people claim that sime Ulama said that chess is allowed. That claim is not correct. Those Ulama never said that chess is always allowed but only if it is used and understood in the society as a mean to activate thinking. That is a conditional permission only under such situation and for those who do their Taqleed.

Every Mo'min after all must follow the verdict of his Marje' of Taqleed.

The question of : Why? Is the same for all the Islamic rules. All Islamic rules are based on the benefit for the human beings. Every forbidden act is very harmful whether we know the details of the harm or not. Why men are not allowed to wear gold or pure silk? Allah and His messenger and Ahlul Bayt know the harm. We believe in whatever Allah ordered the Prophet (SAWA) to convey to us.

People's opinions differ from time to time and due to different reasons, but true religions does not change.

Wassalam.