Are personal business investments tax deductible?

I don't know much about taxes in the first place, and correct me if i am wrong on this, but tax deductions work basically like taking whatever is deductible and subtracting it from your gross income right? That new lowered gross income is now what is taxable. With that understanding of deductions, would a business investment be tax deductible? For example, hypothetically speaking, if I get my paycheck from my employer and then give them back, or "invest", half of that back into the company, would that "investment" be tax deductible for me? So hypothetically if I grossed $100k for the year and gave half each paycheck back to the company, would I only have to pay taxes on $50k? And to go one step further, from the company's point of view, would that $50k investment from me be considered taxable income for the company?