What criteria do we use to decide whether something is alive?

What criteria do we use to decide whether something is alive? In the vast expanse of our universe, the concept of life remains both enigmatic and captivating. From the microscopic realms of single-celled organisms to the vast diversity of complex multicellular beings, the question of what truly defines something as “alive” has puzzled philosophers, scientists, and curious minds alike. While the answer may seem elusive, modern science has established a set of criteria to help us navigate this profound inquiry.

What criteria do we use to decide whether something is alive?

what criteria do we use to decide whether something is alive
what criteria do we use to decide whether something is alive

Determining whether something is alive involves assessing several criteria that biologists typically use. These criteria help differentiate living organisms from non-living entities. Here are the key characteristics commonly used:

  1. Cellular Organization: Living things are composed of cells, which are the basic unit of life. These can be single-celled (like bacteria) or multicellular (like humans and trees).
  2. Metabolism: Living organisms undergo metabolism, which includes all chemical processes that produce energy and enable growth, maintaining their internal environment despite changing external conditions.
  3. Growth: Living things grow by increasing in size or cell number. Growth involves the assimilation and use of nutrients from the environment.
  4. Reproduction: Living organisms have the ability to reproduce, either sexually or asexually, to pass genetic information onto their offspring.
  5. Response to Stimuli: Life forms respond to environmental stimuli through mechanisms that can be as simple as moving towards light or as complex as intricate hormonal responses.
  6. Homeostasis: This is the ability to maintain a constant internal environment in response to environmental changes. For example, humans maintain a relatively constant body temperature.
  7. Evolution: Populations of living organisms evolve over time through changes in their genetic makeup. Evolution is driven by natural selection, ensuring that traits beneficial for survival and reproduction are passed on to succeeding generations.

Not every characteristic must be present at all times but generally, living organisms will exhibit most of these traits. For example, a seed might lie dormant and not show visible growth or response to stimuli, but it retains the potential to do so under the right conditions.

The Seven Pillars of Life

Biologists have identified seven fundamental characteristics that collectively distinguish living organisms from non-living entities. These criteria serve as a framework for understanding the essence of life and its intricate complexities.


Living organisms exhibit a remarkable level of organization, from the intricate arrangement of molecules within cells to the intricate systems that govern their functions. This organized structure reflects the complexity and coordination required for life to thrive.


All living beings possess metabolic processes that enable them to convert nutrients into energy and building blocks for growth, repair, and maintenance. Metabolism is the engine that drives the chemical reactions essential for life.

Growth and Development

Living organisms have the inherent ability to grow and develop, undergoing changes in size, shape, and complexity throughout their lifespans. This characteristic sets them apart from non-living entities, which remain static and unchanging.

Response to Stimuli

Living beings are capable of sensing and responding to various stimuli in their environment, such as light, temperature, or chemical signals. This responsiveness allows them to adapt and survive in changing conditions.


Maintaining a stable internal environment is crucial for life to persist. Living organisms possess homeostatic mechanisms that regulate and maintain essential conditions, such as pH levels, temperature, and nutrient concentrations, within acceptable ranges.


The ability to reproduce and pass on genetic information to offspring is a hallmark of life. This process ensures the continuity of species and allows for the evolution and adaptation of organisms over time.


Living organisms are subject to evolutionary processes, which involve changes in their genetic makeup over generations. This process of evolution is driven by natural selection and enables organisms to adapt to changing environmental conditions, leading to the diversity of life we observe today.

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The Exceptions and Complexities

While these seven criteria provide a general framework for identifying life, the reality is often more nuanced. Certain entities, such as viruses, challenge our conventional understanding of what constitutes a living organism. Additionally, the boundaries between life and non-life can become blurred in extreme environments or at the molecular level, where the line between chemistry and biology becomes increasingly blurred.

The Ongoing Quest for Understanding

As our knowledge of the natural world continues to expand, our understanding of life and its complexities deepens. The criteria for defining life remain a subject of ongoing scientific inquiry and philosophical contemplation. By unraveling the mysteries of life, we not only gain insights into the intricate workings of our world but also uncover profound truths about our place within the vast tapestry of existence.

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