We have the right to life, liberty and the pursuit of happiness.

LIFE

Life - 1 a: the quality that distinguishes a vital and functional being from a dead body b: one or more aspects of the process of living.  the period during which something exists, lasts, or is in progress  http://www.merriam-webster.com/dictionary/life    http://www.merriam-webster.com/thesaurus/life

Since life ends (death) when there is no brain activity and/or if the heart stops beating, then that would be a good indicator that live begins when there is a heart beat and/or brain activity.  An unborn child has a heartbeat almost from conception.

We have always taken "the right to life" to mean "thou shalt not kill." 

It also means you have the right to be born (life). 

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LIBERTY

Liberty - 1: the quality or state of being free: a: the power to do as one pleases b: freedom from physical restraint c: freedom from arbitrary or despotic control d: the positive enjoyment of various social, political, or economic rights and privileges e: the power of choice. the state of being free from the control or power of another  http://www.merriam-webster.com/dictionary/Liberty   http://www.merriam-webster.com/thesaurus/liberty

We are free to do as we please so long as that freedom does not infringe on another's freedoms.  IE We are free to get drunk in our own homes so long as we do not harm anyone while we are drunk.  We are not free to drink and drive because we take a chance that we might infringe on someone else's right to life. 

We do not belong to anyone.  No person can be owned by anyone else because that would infringe on their liberties.  Ergo: from the beginning slavery was unconstitutional.

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PURSUIT OF HAPPINESS

Pursuit - 2: to find or employ measures to obtain or accomplish, Synonyms - chase, chasing, dogging, following, hounding, pursuing, shadowing, tagging, tailing, tracing, tracking, trailing; Related Words - tagging along; path, track, trail; search, seeking  http://www.merriam-webster.com/dictionary/pursuing    http://www.merriam-webster.com/thesaurus/pursuit

Happiness - good fortune : prosperity 2 a: a state of well-being and contentment : joy b: a pleasurable or satisfying experience; Synonyms - blessedness, bliss, blissfulness, felicity, gladness, joy; Related Words - elatedness, elation, exhilaration, exultation, intoxication; ecstasy, euphoria, heaven, rapture, rapturousness; delectation, delight, enjoyment, pleasure; cheer, cheerfulness, exuberance, gaiety (also gayety), gladsomeness, glee, gleefulness, jollity, joyfulness, joyousness, jubilation, lightheartedness; content, contentedness, gratification, satisfaction; http://www.merriam-webster.com/dictionary/happiness     http://www.merriam-webster.com/thesaurus/happiness

We are not guaranteed happiness.  We are guaranteed the right to pursue happiness. 

We have the right to woo, court and propose marriage.  We do not have the right to force marriage. 

We have the right to find and apply for the job of our choice.  We do not have the right to force employment.

We have the right to pursue ways to become wealthy.  We do not have the right to break the law to become rich.

We have the right to pursue the God of our choice.  We do not have the right to force others to believe what we believe. 

We have the right to pursue the house that will be our home.  We do not have the right to take someone else's home.

We have the right to pursue the choice of were we want to live (city, country, seaside, mountains, state).  We do not have the right to force someone to take us there.

In other words this basic right is ours to use responsibly, in such a manner that we do not infringe on another person's rights.