The rule of threes is basically 3 minutes without air, 3 hours without shelter in bad weather, 3 days without water, 3 weeks without food...and they should be the foundation of how you prioritize your prep, everything else is secondary.
That mindset helped when I got more serious about backpacking..planning for water and shelter became the top priority before I even thought about what else to pack.
I find that sticking to the 'rule of threes' really helps you avoid falling for all those gimmicks out there. Always think shelter and water first, long before you even consider any tactical gear.
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