private function validateInput(InputInterface $input)
    {
        if (!$input->getArgument('target')) {
            // if no target is passed, there can be no properties or methods
            foreach (['properties', 'methods', 'no-inherit'] as $option) {
                if ($input->getOption($option)) {
                    throw new RuntimeException('--' . $option . ' does not make sense without a specified target');
                }
            }

            foreach (['globals', 'vars', 'constants', 'functions', 'classes', 'interfaces', 'traits'] as $option) {
                if ($input->getOption($option)) {
                    return;
                }
            }

            // default to --vars if no other options are passed
            $input->setOption('vars', true);
        } else {
            // if a target is passed, classes, functions, etc don't make sense
            foreach (['vars', 'globals', 'functions', 'classes', 'interfaces', 'traits'] as $option) {
                if ($input->getOption($option)) {
                    throw new RuntimeException('--' . $option . ' does not make sense with a specified target');
                }
            }

            foreach (['constants', 'properties', 'methods'] as $option) {
                if ($input->getOption($option)) {
                    return;
                }
            }

            // default to --constants --properties --methods if no other options are passed
            $input->setOption('constants',  true);
            $input->setOption('properties', true);
            $input->setOption('methods',    true);
        }
    }