The ways a creature can use Perception depend on what senses it has. The primary concepts you need to know for understanding senses are precise senses, imprecise senses, and the three states of detection a target can be in: observed, hidden, or undetected. Vision, hearing, and scent are three prominent senses, but they don’t have the same degree of acuity.
Precise Senses
Average vision is a precise sense—a sense that can be used to perceive the world in nuanced detail. The only way to target a creature without having drawbacks is to use a precise sense. You can usually detect a creature automatically with a precise sense unless that creature is hiding or obscured by the environment, in which case you can use the Seek basic action to better detect the creature.
Imprecise Senses
Hearing is an imprecise sense—it cannot detect the full range of detail that a precise sense can. You can usually sense a creature automatically with an imprecise sense, but it has the hidden condition instead of the observed condition. It might be undetected by you if it’s using Stealth or is in an environment that distorts the sense, such as a noisy room in the case of hearing. In those cases, you have to use the Seek basic action to detect the creature. At best, an imprecise sense can be used to make an undetected creature (or one you didn’t even know was there) merely hidden—it can’t make the creature observed.
Vague Senses
A character also has many vague senses—ones that can alert you that something is there but aren’t useful for zeroing in on it to determine exactly what it is. The most useful of these for a typical character is the sense of smell. At best, a vague sense can be used to detect the presence of an unnoticed creature, making it undetected. Even then, the vague sense isn’t sufficient to make the creature hidden or observed.
When one creature might detect another, the GM almost always uses the most precise sense available.
Pathfinder’s rules assume that a given creature has vision as its only precise sense and hearing as its only imprecise sense. Some characters and creatures, however, have precise or imprecise senses that don’t match this assumption. For instance, a character with poor vision might treat that sense as imprecise, an animal with the scent ability can use its sense of smell as an imprecise sense, and a creature with echolocation or a similar ability can use hearing as a precise sense. Such senses are often given special names and appear as “echolocation (precise),” “scent (imprecise) 30 feet,” or the like.
Detecting with Other Senses
If a monster uses a sense other than vision, the GM can adapt the variables that keep its foes from being detected to equivalents that work with the monster’s senses. For example, a creature that has echolocation might use hearing as a primary sense. This could mean its quarry is concealed in a noisy chamber, hidden in a great enough din, or even invisible in the area of a
spell.Using Stealth with Other Senses
The Stealth skill is designed to use Hide for avoiding visual detection and Avoid Notice and Sneak to avoid being both seen and heard. For many special senses, a player can describe how they’re avoiding detection by that special sense and use the most applicable Stealth action. For instance, a creature stepping lightly to avoid being detected via tremorsense would be using Sneak.
In some cases, rolling a Dexterity-based Stealth skill check to Sneak doesn’t make the most sense. For example, when facing a creature that can detect heartbeats, a PC trying to avoid being detected might meditate to slow their heart rate, using Wisdom instead of Dexterity as the ability modifier for the Stealth check. When a creature that can detect you has multiple senses, such as if it could also hear or see, the PC would use the lowest applicable ability modifier for the check.
Special Senses
While a human might have a difficult time making creatures out in dim light, an elf can see those creatures just fine. And though elves have no problem seeing on a moonlit night, their vision cannot penetrate complete darkness, whereas a dwarf’s can.
Special senses grant greater awareness that allows a creature with these senses to either ignore or reduce the effects of the undetected, hidden, or concealed conditions (described in Detecting Creatures below) when it comes to situations that foil average vision. The following are a few examples of common special senses.
Darkvision and Greater Darkvision
A creature with darkvision or greater darkvision can see perfectly well in areas of darkness and dim light, though such vision is in black and white only. Some forms of magical darkness, such as a 4th-level
spell, block normal darkvision. A creature with greater darkvision, however, can see through even these forms of magical darkness.Low-Light Vision
A creature with low-light vision can see in dim light as though it were bright light, so it ignores the concealed condition due to dim light.
Scent
Scent involves sensing creatures or objects by smell, and is usually a vague sense. The range is listed in the ability, and it functions only if the creature or object being detected emits an aroma (for instance, incorporeal creatures usually do not exude an aroma).
If a creature emits a heavy aroma or is upwind, the GM can double or even triple the range of scent abilities used to detect that creature, and the GM can reduce the range if a creature is downwind.
Tremorsense
Tremorsense allows a creature to feel the vibrations through a solid surface caused by movement. It is usually an imprecise sense with a limited range (listed in the ability). Tremorsense functions only if the detecting creature is on the same surface as the subject, and only if the subject is moving along (or burrowing through) the surface.