sex, gender, nature
性 centers on the inherent qualities that define a being: its sex or gender, and its fundamental nature or disposition.
性 combines 忄 (heart/mind) with 生, which likely contributes both sound and the idea of life or birth, suggesting the innate qualities one is born with.
The heart radical 忄 beside life 生 points to the inner, inborn qualities of a living being: its sex and its nature.
For セイ, imagine a sage explaining the nature of all things: sage -> セイ, and the sage's wisdom reveals the true nature of life.
nature (of a person)
woman; female
man; male
character (of a person); personality; disposition; nature
ability; performance; efficiency
affinity; compatibility; chemistry (between people)
individuality; personality; quirk; idiosyncrasy; character; individual characteristic
sexual desire; sex drive; lust
activity
nature (of a person); disposition; temperament; character
(distinction of) sex; gender
sexual (i.e. relating to biological sex)
special characteristic; special quality; trait; idiosyncrasy; peculiarity
the opposite sex
intelligence
homosexuality; homosexual love
reason; reasoning power; (one's) sense
resistance (to heat, corrosion, etc.); tolerance (to a drug, pain, etc.)
disposition; inclination; characteristic; idiosyncrasy; propensity
high efficiency; high performance
willpower; guts; determination; grit; spirit
sensitivity; sensitiveness; sense (of ...)
same sex
sexual intercourse; sex; coitus
chronic (illness)
true character; real nature
acute (illness)
positive (result)
neutrality (incl. chemical, electrical, etc.); indifference
aptitude; aptness; suitability
toxicity; virulence
malignant (cancer); virulent; pernicious (anemia, anaemia)
sexually transmitted disease; STD; sexually transmitted infection; STI; venereal disease; VD
motherhood; maternity
habit
acidity
sensitivity; sensibility; susceptibility; receptivity
very much; intensely; overwhelmingly; excessively; irresistibly; without any obvious reason
negative (result)
disposition; temperament; temper; nature
wildness; wild nature; untamed nature; savage nature
sex education
nature; natural constitution; innate disposition
neutron
alkalinity
sensitivity to cold; poor circulation
force of habit
nature; disposition
character; personality
inclination; tendency; nature; character
hasty; rash; rushed; hurried; impatient; impetuous; quick-tempered
tendency to become totally absorbed in something; monomania; perfectionism; fastidiousness; meticulousness
character and conduct
humanity; human nature
credibility; authenticity; confidence; reliability
individual; distinctive; unique; characteristic; personal; idiosyncratic
(degree of) risk; riskiness; danger
combustibility; inflammability
possibility; chance; likelihood; probability
very likely; very probable
national traits; national characteristic; character of the inhabitants of a nation
diversity; variety
feminine; effeminate
future prospects; promise
necessity; need
erogenous zone
chronic
radioactive
water-soluble
productivity
manly
sociality
senile (dementia, cataract, etc.)
durability
importance; gravity