The only business expenses that can be deducted against your income so as to arrive at your taxable income, are expenses incurred wholly and exclusively in generating that business income. Any non-business / personal expenses are not tax deductible. Also, any expenses of a capital nature are also not tax deductible.
For instance, if you are an IT consultant who works from home, a portion of your rent can be deemed as a business expense. Further, a portion of your internet bill, anti-virus expenses, city council licenses and telephone bills, can be deemed to be a tax deductible business expense.
However, school fees for the IT professional’s children is not a tax deductible business expense as it is personal in nature, in the same way the purchase of a laptop is not tax deductible as it is capital in nature.