Sentence:  This notion of ‘<WSD>ulterior</WSD> intent' is the closest legal term to the concept of ‘motive' as both share the character of hiddenness. In terms of a reason or motive for an action, you don't hide something unless you really want (to do) something. So, if your reason or motive for an action is characterized or called as ‘<WSD>ulterior</WSD>', it indicates that what you secretly want cannot be an ‘unwanted (or uninterested) but permitted side-effect'. Instead, it must be the ‘desired main effect' of your <WSD>ulterior</WSD> motive or intent. 
Meaning of the word marked with <WSD> tags:  Beyond what is obvious or evident.