Mr. Apple says... 

> Object properties [are strong by default](https://developer.apple.com/library/mac/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html); the strong attribute is described in “[ARC Introduces New Lifetime Qualifiers.”](https://developer.apple.com/library/mac/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW4)