You have two additional “defenses” based on skills, which are used to determine whether you can see past attempts at stealth, illusion or deceit. Your Spot defense is equal to 10 + your Perception skill check modifier, and your Sense defense is equal to 10 + your Insight skill check modifier. Many skill effects, such as Stealth and Bluff, target your Spot or Sense defense, and these defenses determine how likely you are to notice particular features of any area that you are in.