block里为什么不能操作成员变量？

我想在一个block里对一个成员变量赋值，但是提示“ use of undeclared identifier self"错误，
请问这是怎么回事？

void (^successBlock)(id ) = ^(id responseObject){
    NSLog(@"responseObject:%@",responseObject);
    NSInteger result = [[responseObject objectForKey:@"result"]integerValue];
    if(result == 1){
        self.missionsArr= [[responseObject objectForKey:@"data"]objectForKey:@"list"];
    }else if (result == 0){
        NSLog(@"0");
    }
};