Aroma vs heat
Spice has two separate layers: what you smell, and what you feel as bite, and both are worth balancing.
Also: flavour vs pungency, heat versus flavour, sensory dimensions
Spices act through several senses. Aroma reaches the nose, both through the air and from the back of the mouth, and gives flavour such as sweet, floral or smoky. Pungency is felt by touch and pain receptors and gives heat, tingle or numbness. Taste proper, sweet, sour, salty, bitter and umami, comes from the tongue. Chillies show the split well: Kashmiri chilli is colourful and gentle, a habanero is fruity and fierce. A good cook chooses spices for each layer, such as cumin for aroma, chilli for heat and tamarind for sour. Heat without aroma tastes flat, and aroma without heat can seem timid.